feat: add the .opt_css() method

[rich-iannone]

This PR adds the .opt_css() method. It allows users to add custom CSS to HTML-output tables. This opt_*() method builds on the existing functionality of the tab_options(table_additional_css=) option by allowing users to more easily and safely append CSS rulesets to that Options parameter.

from great_tables import GT, exibble
import polars as pl

exibble_mini = pl.from_pandas(exibble).select(["num", "currency"])

(
    GT(exibble_mini, id="one")
    .fmt_currency(columns="currency", currency="HKD")
    .fmt_scientific(columns="num")
    .opt_css(
        css='''
        #one .gt_table {
          background-color: skyblue;
        }
        #one .gt_row {
          padding: 20px 30px;
        }
        #one .gt_col_heading {
          text-align: center !important;
        }
        '''
    )
)

Notice in that rendered table that: (1) body cells get a 'skyblue' background color, (2) the padding of body cells is increased horizontally and vertically, and (3) column-label text is now centered instead of being right-aligned.

Fixes: #174

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.67%. Comparing base (de3580d) to head (7e6ab01).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #775      +/-   ##
==========================================
+ Coverage   91.61%   91.67%   +0.05%     
==========================================
  Files          47       47              
  Lines        5773     5790      +17     
==========================================
+ Hits         5289     5308      +19     
+ Misses        484      482       -2     

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[machow]

Code review (updated)

MICHAEL NOTES:

• None of these issues seems too bad

Found 5 issues (note: all scored below 80% confidence threshold, so these are minor):

1. Test functions test multiple behaviors (CLAUDE.md says "Test one behavior per case") - Score: 75

   test_opt_css_basic() tests both basic CSS addition AND chaining; test_opt_css_duplicate_handling() tests both default duplicate prevention AND allowing duplicates.

   great-tables/tests/test_options.py

   Lines 553 to 579 in 9fd4016

   	def test_opt_css_basic():
   	# Test adding CSS
   	res = GT(exibble).opt_css(css=".gt_table { background-color: red; }")
   	assert res._options.table_additional_css.value == [".gt_table { background-color: red; }"]
   	# Test chaining CSS additions
   	res_2 = res.opt_css(css=".gt_row { color: blue; }")
   	assert len(res_2._options.table_additional_css.value) == 2
   	assert ".gt_table { background-color: red; }" in res_2._options.table_additional_css.value
   	assert ".gt_row { color: blue; }" in res_2._options.table_additional_css.value
   	def test_opt_css_duplicate_handling():
   	gt_tbl = GT(exibble)
   	css_rule = ".gt_table { color: green; }"
   	# Add same CSS twice (should only appear once)
   	res = gt_tbl.opt_css(css=css_rule).opt_css(css=css_rule)
   	assert res._options.table_additional_css.value == [css_rule]
   	# Test for allowing duplicates
   	res_2 = gt_tbl.opt_css(css=css_rule).opt_css(css=css_rule, allow_duplicates=True)
   	assert res_2._options.table_additional_css.value == [css_rule, css_rule]

2. Tests don't use kwargs dict pattern (CLAUDE.md says "Use kwargs dicts, only specify non-defaults") - Score: 75

   Tests use direct method calls instead of @pytest.mark.parametrize with kwargs dicts pattern.

   great-tables/tests/test_options.py

   Lines 553 to 661 in 9fd4016

   	def test_opt_css_basic():
   	# Test adding CSS
   	res = GT(exibble).opt_css(css=".gt_table { background-color: red; }")
   	assert res._options.table_additional_css.value == [".gt_table { background-color: red; }"]
   	# Test chaining CSS additions
   	res_2 = res.opt_css(css=".gt_row { color: blue; }")
   	assert len(res_2._options.table_additional_css.value) == 2
   	assert ".gt_table { background-color: red; }" in res_2._options.table_additional_css.value
   	assert ".gt_row { color: blue; }" in res_2._options.table_additional_css.value
   	def test_opt_css_duplicate_handling():
   	gt_tbl = GT(exibble)
   	css_rule = ".gt_table { color: green; }"
   	# Add same CSS twice (should only appear once)
   	res = gt_tbl.opt_css(css=css_rule).opt_css(css=css_rule)
   	assert res._options.table_additional_css.value == [css_rule]
   	# Test for allowing duplicates
   	res_2 = gt_tbl.opt_css(css=css_rule).opt_css(css=css_rule, allow_duplicates=True)
   	assert res_2._options.table_additional_css.value == [css_rule, css_rule]
   	def test_opt_css_replace_mode():
   	# Add initial CSS
   	res = (
   	GT(exibble).opt_css(css=".gt_table { color: red; }").opt_css(css=".gt_row { color: blue; }")
   	)
   	assert len(res._options.table_additional_css.value) == 2
   	# Replace all CSS with `add=False`
   	res_2 = res.opt_css(css=".gt_table { color: green; }", add=False)
   	assert res_2._options.table_additional_css.value == [".gt_table { color: green; }"]
   	def test_opt_css_whitespace_handling():
   	gt_tbl = GT(exibble)
   	# Test using empty-string CSS
   	res = gt_tbl.opt_css(css="")
   	assert res._options.table_additional_css.value == []
   	# Test whitespace-only CSS
   	res_2 = gt_tbl.opt_css(css=" \n \t ")
   	assert res_2._options.table_additional_css.value == []
   	# Test CSS with leading and trailing whitespace (it should get stripped)
   	res_3 = gt_tbl.opt_css(css=" .gt_table { color: red; } ")
   	assert res_3._options.table_additional_css.value == [".gt_table { color: red; }"]
   	def test_opt_css_multiline():
   	multiline_css = """
   	#test_table .gt_table {
   	background-color: skyblue;
   	}
   	#test_table .gt_row {
   	padding: 20px;
   	}"""
   	result = GT(exibble, id="test_table").opt_css(css=multiline_css)
   	expected_css = multiline_css.strip()
   	assert result._options.table_additional_css.value == [expected_css]
   	def test_opt_css_html_output():
   	css_rule = "#test_table .gt_table { background-color: lightblue; }"
   	res = GT(exibble, id="test_table").opt_css(css=css_rule)
   	html = res.as_raw_html()
   	# Check that the CSS appears in the HTML string
   	assert css_rule in html
   	# For the CSS ordering, the added CSS should come after the default CSS
   	default_css_pos = html.find("#test_table .gt_table { display: table;")
   	custom_css_pos = html.find(css_rule)
   	assert default_css_pos > 0
   	assert custom_css_pos > 0
   	assert custom_css_pos > default_css_pos
   	def test_opt_css_with_tab_options():
   	res = (
   	GT(exibble)
   	.tab_options(table_additional_css=".initial { color: red; }")
   	.opt_css(css=".added { color: blue; }")
   	)
   	css_list = res._options.table_additional_css.value
   	assert len(css_list) == 2
   	assert ".initial { color: red; }" in css_list
   	assert ".added { color: blue; }" in css_list
   	# Test that `opt_css()` rule comes after the `tab_options()` rule in the rendered HTML
   	html = res.as_raw_html()
   	assert html.index(".added { color: blue; }") > html.index(".initial { color: red; }")

3. Empty string not filtered in tab_options() normalization - Score: 50

   When tab_options(table_additional_css="") is called, it becomes [""] instead of []. This is a rare edge case with minimal impact (extra newline in CSS output).

   great-tables/great_tables/_options.py

   Lines 556 to 559 in 9fd4016

   	# - `table_additional_css` should be a list but if given as a string, ensure it is list
   	if "table_additional_css" in modified_args:
   	if isinstance(modified_args["table_additional_css"], str):
   	modified_args["table_additional_css"] = [modified_args["table_additional_css"]]

4. Comment capitalization inconsistency - Score: 50

   One comment starts with lowercase (# if CSS already exists...) while all others in the function start with capitals.

   great-tables/great_tables/_options.py

   Line 1519 in 9fd4016

   # Use tab_options() to set the additional CSS

5. Missing test coverage for edge cases - Score: 75

   No explicit tests for: opt_css(css="", add=False) clearing existing CSS, or opt_css(css="") preserving existing CSS.

   great-tables/tests/test_options.py

   Lines 599 to 610 in 9fd4016

   	res = gt_tbl.opt_css(css="")
   	assert res._options.table_additional_css.value == []
   	# Test whitespace-only CSS
   	res_2 = gt_tbl.opt_css(css=" \n \t ")
   	assert res_2._options.table_additional_css.value == []
   	# Test CSS with leading and trailing whitespace (it should get stripped)
   	res_3 = gt_tbl.opt_css(css=" .gt_table { color: red; } ")
   	assert res_3._options.table_additional_css.value == [".gt_table { color: red; }"]

---

All issues are below the 80% confidence threshold for reporting as definite problems. The implementation is solid overall.

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