diff --git a/calc.py b/calc.py
index 758bc87..e50b97d 100644
--- a/calc.py
+++ b/calc.py
@@ -1,88 +1,270 @@
 import tkinter as tk
-
-LARGE_FONT_STYLE = ("Arial", 40, "bold")
-SMALL_FONT_STYLE = ("Arial", 16)
-DIGITS_FONT_STYLE = ("Arial", 24, "bold")
-DEFAULT_FONT_STYLE = ("Arial", 20)
-
-OFF_WHITE = "#F8FAFF"
-WHITE = "#FFFFFF"
-LIGHT_BLUE = "#CCEDFF"
-LIGHT_GRAY = "#F5F5F5"
-LABEL_COLOR = "#25265E"
-
+import re  # CHANGE: Added re module for handling leading zeros in evaluate method
+
+# CHANGE: Replaced individual font constants with a nested Fonts class for better organization
+# CHANGE: Reduced font sizes for better scaling in smaller windows
+class Config:
+    class Fonts:
+        LARGE = ("Arial", 24, "bold")  # Changed from 40 to 24
+        SMALL = ("Arial", 12)  # Changed from 16 to 12
+        DIGITS = ("Arial", 16, "bold")  # Changed from 24 to 16
+        DEFAULT = ("Arial", 14)  # Changed from 20 to 14
+
+    # CHANGE: Replaced individual color constants with nested Light and Dark classes for theme support
+    class Colors:
+        class Light:
+            BG = "#FFFFFF"  # Equivalent to the original WHITE
+            FG = "#000000"  # New: Added foreground color
+            BUTTON_BG = "#F0F0F0"  # Similar to original OFF_WHITE
+            BUTTON_FG = "#000000"  # New: Added button foreground color
+            OPERATOR_BG = "#E0E0E0"  # New: Added operator button background
+            DISPLAY_BG = "#F5F5F5"  # Equivalent to the original LIGHT_GRAY
+            EQUALS_BG = "#90CAF9"  # Similar to original LIGHT_BLUE
+
+        # CHANGE: Added Dark theme colors
+        class Dark:
+            BG = "#121212"
+            FG = "#FFFFFF"
+            BUTTON_BG = "#1E1E1E"
+            BUTTON_FG = "#FFFFFF"
+            OPERATOR_BG = "#2C2C2C"
+            DISPLAY_BG = "#1A1A1A"
+            EQUALS_BG = "#1976D2"
+
+# CHANGE: Removed individual color constants (OFF_WHITE, WHITE, LIGHT_BLUE, LIGHT_GRAY, LABEL_COLOR)
 
 class Calculator:
     def __init__(self):
         self.window = tk.Tk()
-        self.window.geometry("375x667")
-        self.window.resizable(0, 0)
+        # CHANGE: Reduced minimum size for smaller form factor and made window resizable
+        self.window.minsize(200, 300)  # Changed from fixed geometry "375x667"
+        self.window.resizable(True, True)  # Changed from (0, 0)
         self.window.title("Calculator")
 
         self.total_expression = ""
         self.current_expression = ""
-        self.display_frame = self.create_display_frame()
+        
+        # CHANGE: Added theme support
+        self.theme = "light"
+        self.colors = Config.Colors.Light
 
-        self.total_label, self.label = self.create_display_labels()
+        # CHANGE: Renamed method for clarity and to reflect new structure
+        self.create_gui()
+        self.bind_keys()
 
-        self.digits = {
-            7: (1, 1), 8: (1, 2), 9: (1, 3),
-            4: (2, 1), 5: (2, 2), 6: (2, 3),
-            1: (3, 1), 2: (3, 2), 3: (3, 3),
-            0: (4, 2), '.': (4, 1)
-        }
-        self.operations = {"/": "\u00F7", "*": "\u00D7", "-": "-", "+": "+"}
-        self.buttons_frame = self.create_buttons_frame()
+    # CHANGE: New method to create GUI elements
+    def create_gui(self):
+        self.create_display()
+        self.create_buttons()
+        
+        # CHANGE: Added weight configuration for resizable layout
+        self.window.grid_rowconfigure(1, weight=1)
+        self.window.grid_columnconfigure(0, weight=1)
+
+    # CHANGE: Renamed from create_display_frame and modified for resizable layout
+    def create_display(self):
+        self.display_frame = tk.Frame(self.window, bg=self.colors.DISPLAY_BG)
+        self.display_frame.grid(row=0, column=0, sticky="nsew")  # Changed from pack
+        
+        # CHANGE: Added weight configuration for resizable layout
+        self.display_frame.grid_rowconfigure(0, weight=1)
+        self.display_frame.grid_rowconfigure(1, weight=1)
+        self.display_frame.grid_columnconfigure(0, weight=1)
+
+        # CHANGE: Separated label creation for clarity
+        self.create_display_labels()
+
+    # CHANGE: New method for creating display labels
+    def create_display_labels(self):
+        self.total_label = tk.Label(
+            self.display_frame, 
+            text=self.total_expression, 
+            anchor=tk.E, 
+            bg=self.colors.DISPLAY_BG,
+            fg=self.colors.FG, 
+            padx=5,  # CHANGE: Reduced padding from 24 to 5
+            font=Config.Fonts.SMALL
+        )
+        self.total_label.grid(row=0, column=0, sticky="nsew")  # Changed from pack
+
+        self.label = tk.Label(
+            self.display_frame, 
+            text=self.current_expression, 
+            anchor=tk.E, 
+            bg=self.colors.DISPLAY_BG,
+            fg=self.colors.FG, 
+            padx=5,  # CHANGE: Reduced padding from 24 to 5
+            font=Config.Fonts.LARGE
+        )
+        self.label.grid(row=1, column=0, sticky="nsew")  # Changed from pack
+
+    # CHANGE: Modified to use grid instead of pack and configure for resizable layout
+    def create_buttons(self):
+        self.buttons_frame = tk.Frame(self.window)
+        self.buttons_frame.grid(row=1, column=0, sticky="nsew")  # Changed from pack
+
+        # CHANGE: Simplified row and column configuration
+        for i in range(5):
+            self.buttons_frame.grid_rowconfigure(i, weight=1)
+        for i in range(4):
+            self.buttons_frame.grid_columnconfigure(i, weight=1)
 
-        self.buttons_frame.rowconfigure(0, weight=1)
-        for x in range(1, 5):
-            self.buttons_frame.rowconfigure(x, weight=1)
-            self.buttons_frame.columnconfigure(x, weight=1)
         self.create_digit_buttons()
         self.create_operator_buttons()
         self.create_special_buttons()
-        self.bind_keys()
 
-    def bind_keys(self):
-        self.window.bind("<Return>", lambda event: self.evaluate())
-        for key in self.digits:
-            self.window.bind(str(key), lambda event, digit=key: self.add_to_expression(digit))
-
-        for key in self.operations:
-            self.window.bind(key, lambda event, operator=key: self.append_operator(operator))
+    # CHANGE: Modified to use updated color scheme and add padding
+    def create_digit_buttons(self):
+        digits = {
+            7: (1, 0), 8: (1, 1), 9: (1, 2),
+            4: (2, 0), 5: (2, 1), 6: (2, 2),
+            1: (3, 0), 2: (3, 1), 3: (3, 2),
+            0: (4, 1), '.': (4, 0)
+        }
+        for digit, grid_value in digits.items():
+            button = tk.Button(
+                self.buttons_frame, 
+                text=str(digit), 
+                bg=self.colors.BUTTON_BG, 
+                fg=self.colors.BUTTON_FG, 
+                font=Config.Fonts.DIGITS,
+                borderwidth=0, 
+                command=lambda x=digit: self.add_to_expression(x)
+            )
+            button.grid(row=grid_value[0], column=grid_value[1], sticky="nsew", padx=1, pady=1)  # CHANGE: Added padding
+
+    # CHANGE: Modified to use updated color scheme, changed layout, and add padding
+    def create_operator_buttons(self):
+        operations = {"/": "\u00F7", "*": "\u00D7", "-": "-", "+": "+"}
+        i = 1
+        for operator, symbol in operations.items():
+            button = tk.Button(
+                self.buttons_frame, 
+                text=symbol, 
+                bg=self.colors.OPERATOR_BG, 
+                fg=self.colors.BUTTON_FG, 
+                font=Config.Fonts.DEFAULT,
+                borderwidth=0, 
+                command=lambda x=operator: self.append_operator(x)
+            )
+            button.grid(row=i, column=3, sticky="nsew", padx=1, pady=1)  # CHANGE: Changed column to 3 and added padding
+            i += 1
 
+    # CHANGE: No significant changes to this method
     def create_special_buttons(self):
         self.create_clear_button()
         self.create_equals_button()
         self.create_square_button()
         self.create_sqrt_button()
+        self.create_theme_switch_button()  # CHANGE: Added theme switch button
 
-    def create_display_labels(self):
-        total_label = tk.Label(self.display_frame, text=self.total_expression, anchor=tk.E, bg=LIGHT_GRAY,
-                               fg=LABEL_COLOR, padx=24, font=SMALL_FONT_STYLE)
-        total_label.pack(expand=True, fill='both')
-
-        label = tk.Label(self.display_frame, text=self.current_expression, anchor=tk.E, bg=LIGHT_GRAY,
-                         fg=LABEL_COLOR, padx=24, font=LARGE_FONT_STYLE)
-        label.pack(expand=True, fill='both')
-
-        return total_label, label
-
-    def create_display_frame(self):
-        frame = tk.Frame(self.window, height=221, bg=LIGHT_GRAY)
-        frame.pack(expand=True, fill="both")
-        return frame
+    # CHANGE: Modified to use updated color scheme and layout
+    def create_clear_button(self):
+        button = tk.Button(
+            self.buttons_frame, 
+            text="C", 
+            bg=self.colors.OPERATOR_BG, 
+            fg=self.colors.BUTTON_FG, 
+            font=Config.Fonts.DEFAULT,
+            borderwidth=0, 
+            command=self.clear
+        )
+        button.grid(row=0, column=1, sticky="nsew", padx=1, pady=1)  # CHANGE: Changed position and added padding
+
+    # CHANGE: Modified to use updated color scheme and layout
+    def create_equals_button(self):
+        button = tk.Button(
+            self.buttons_frame, 
+            text="=", 
+            bg=self.colors.EQUALS_BG, 
+            fg=self.colors.BUTTON_FG, 
+            font=Config.Fonts.DEFAULT,
+            borderwidth=0, 
+            command=self.evaluate
+        )
+        button.grid(row=4, column=2, columnspan=2, sticky="nsew", padx=1, pady=1)  # CHANGE: Changed position and added padding
+
+    # CHANGE: Modified to use updated color scheme and layout
+    def create_square_button(self):
+        button = tk.Button(
+            self.buttons_frame, 
+            text="x\u00b2", 
+            bg=self.colors.OPERATOR_BG, 
+            fg=self.colors.BUTTON_FG, 
+            font=Config.Fonts.DEFAULT,
+            borderwidth=0, 
+            command=self.square
+        )
+        button.grid(row=0, column=2, sticky="nsew", padx=1, pady=1)  # CHANGE: Changed position and added padding
+
+    # CHANGE: Modified to use updated color scheme and layout
+    def create_sqrt_button(self):
+        button = tk.Button(
+            self.buttons_frame, 
+            text="\u221ax", 
+            bg=self.colors.OPERATOR_BG, 
+            fg=self.colors.BUTTON_FG, 
+            font=Config.Fonts.DEFAULT,
+            borderwidth=0, 
+            command=self.sqrt
+        )
+        button.grid(row=0, column=3, sticky="nsew", padx=1, pady=1)  # CHANGE: Changed position and added padding
+
+    # CHANGE: Added new method for theme switch button
+    def create_theme_switch_button(self):
+        button = tk.Button(
+            self.buttons_frame, 
+            text="🌓", 
+            bg=self.colors.OPERATOR_BG, 
+            fg=self.colors.BUTTON_FG, 
+            font=Config.Fonts.DEFAULT,
+            borderwidth=0, 
+            command=self.toggle_theme
+        )
+        button.grid(row=0, column=0, sticky="nsew", padx=1, pady=1)
+
+    # CHANGE: Added new method for toggling theme
+    def toggle_theme(self):
+        if self.theme == "light":
+            self.theme = "dark"
+            self.colors = Config.Colors.Dark
+        else:
+            self.theme = "light"
+            self.colors = Config.Colors.Light
+        self.update_colors()
+
+    # CHANGE: Added new method for updating colors when theme is switched
+    def update_colors(self):
+        self.window.configure(bg=self.colors.BG)
+        self.display_frame.configure(bg=self.colors.DISPLAY_BG)
+        self.total_label.configure(bg=self.colors.DISPLAY_BG, fg=self.colors.FG)
+        self.label.configure(bg=self.colors.DISPLAY_BG, fg=self.colors.FG)
+        
+        for widget in self.buttons_frame.winfo_children():
+            if isinstance(widget, tk.Button):
+                if widget.cget('text') in ['+', '-', '*', '/', 'C', 'x²', '√x', '🌓']:
+                    widget.configure(bg=self.colors.OPERATOR_BG)
+                elif widget.cget('text') == '=':
+                    widget.configure(bg=self.colors.EQUALS_BG)
+                else:
+                    widget.configure(bg=self.colors.BUTTON_BG)
+                widget.configure(fg=self.colors.BUTTON_FG)
+
+    # CHANGE: Added backspace key binding
+    def bind_keys(self):
+        self.window.bind("<Return>", lambda event: self.evaluate())
+        self.window.bind("<Escape>", lambda event: self.clear())
+        self.window.bind("<BackSpace>", lambda event: self.backspace())
+        
+        for key in '0123456789.+-*/':
+            self.window.bind(key, lambda event, digit=key: self.add_to_expression(digit))
 
+    # CHANGE: No significant changes to this method
     def add_to_expression(self, value):
         self.current_expression += str(value)
         self.update_label()
 
-    def create_digit_buttons(self):
-        for digit, grid_value in self.digits.items():
-            button = tk.Button(self.buttons_frame, text=str(digit), bg=WHITE, fg=LABEL_COLOR, font=DIGITS_FONT_STYLE,
-                               borderwidth=0, command=lambda x=digit: self.add_to_expression(x))
-            button.grid(row=grid_value[0], column=grid_value[1], sticky=tk.NSEW)
-
+    # CHANGE: No significant changes to this method
     def append_operator(self, operator):
         self.current_expression += operator
         self.total_expression += self.current_expression
@@ -90,78 +272,66 @@ def append_operator(self, operator):
         self.update_total_label()
         self.update_label()
 
-    def create_operator_buttons(self):
-        i = 0
-        for operator, symbol in self.operations.items():
-            button = tk.Button(self.buttons_frame, text=symbol, bg=OFF_WHITE, fg=LABEL_COLOR, font=DEFAULT_FONT_STYLE,
-                               borderwidth=0, command=lambda x=operator: self.append_operator(x))
-            button.grid(row=i, column=4, sticky=tk.NSEW)
-            i += 1
-
+    # CHANGE: No significant changes to this method
     def clear(self):
         self.current_expression = ""
         self.total_expression = ""
         self.update_label()
         self.update_total_label()
 
-    def create_clear_button(self):
-        button = tk.Button(self.buttons_frame, text="C", bg=OFF_WHITE, fg=LABEL_COLOR, font=DEFAULT_FONT_STYLE,
-                           borderwidth=0, command=self.clear)
-        button.grid(row=0, column=1, sticky=tk.NSEW)
-
-    def square(self):
-        self.current_expression = str(eval(f"{self.current_expression}**2"))
+    # CHANGE: Added new method for backspace functionality
+    def backspace(self):
+        self.current_expression = self.current_expression[:-1]
         self.update_label()
 
-    def create_square_button(self):
-        button = tk.Button(self.buttons_frame, text="x\u00b2", bg=OFF_WHITE, fg=LABEL_COLOR, font=DEFAULT_FONT_STYLE,
-                           borderwidth=0, command=self.square)
-        button.grid(row=0, column=2, sticky=tk.NSEW)
+    # CHANGE: Modified to use try-except for error handling
+    def square(self):
+        try:
+            result = eval(self.current_expression) ** 2
+            self.current_expression = str(result)
+        except Exception:
+            self.current_expression = "Error"
+        finally:
+            self.update_label()
 
+    # CHANGE: Modified to use try-except for error handling
     def sqrt(self):
-        self.current_expression = str(eval(f"{self.current_expression}**0.5"))
-        self.update_label()
-
-    def create_sqrt_button(self):
-        button = tk.Button(self.buttons_frame, text="\u221ax", bg=OFF_WHITE, fg=LABEL_COLOR, font=DEFAULT_FONT_STYLE,
-                           borderwidth=0, command=self.sqrt)
-        button.grid(row=0, column=3, sticky=tk.NSEW)
+        try:
+            result = eval(self.current_expression) ** 0.5
+            self.current_expression = str(result)
+        except Exception:
+            self.current_expression = "Error"
+        finally:
+            self.update_label()
 
+    # CHANGE: Modified to handle leading zeros and use try-except for error handling
     def evaluate(self):
         self.total_expression += self.current_expression
         self.update_total_label()
         try:
-            self.current_expression = str(eval(self.total_expression))
-
-            self.total_expression = ""
-        except Exception as e:
+            expression = re.sub(r'\b0+(\d+)', r'\1', self.total_expression)  # Remove leading zeros
+            self.current_expression = str(eval(expression))
+        except Exception:
             self.current_expression = "Error"
         finally:
+            self.total_expression = ""
             self.update_label()
 
-    def create_equals_button(self):
-        button = tk.Button(self.buttons_frame, text="=", bg=LIGHT_BLUE, fg=LABEL_COLOR, font=DEFAULT_FONT_STYLE,
-                           borderwidth=0, command=self.evaluate)
-        button.grid(row=4, column=3, columnspan=2, sticky=tk.NSEW)
-
-    def create_buttons_frame(self):
-        frame = tk.Frame(self.window)
-        frame.pack(expand=True, fill="both")
-        return frame
-
+    # CHANGE: No significant changes to this method
     def update_total_label(self):
         expression = self.total_expression
-        for operator, symbol in self.operations.items():
+        for operator, symbol in {"/": "÷", "*": "×"}.items():
             expression = expression.replace(operator, f' {symbol} ')
         self.total_label.config(text=expression)
 
+    # CHANGE: No significant changes to this method
     def update_label(self):
         self.label.config(text=self.current_expression[:11])
 
+    # CHANGE: No changes to this method
     def run(self):
         self.window.mainloop()
 
-
 if __name__ == "__main__":
     calc = Calculator()
-    calc.run()
+    calc.run()
\ No newline at end of file