Add dark mode support

[RKBoss6]

In conjunction with #4103, this helps add dark mode to the app loader. Adding dark mode helps significantly with eye strain, and blending into the device's UI. Check it out on my personal app loader! and let me know of any suggestions/comments.

Specifically, this PR handles ensuring the right theme is displayed, and theming of app html customizer iframes.

Screenshots:

[Copilot]

Pull request overview

This PR adds dark mode support to the app loader with three theme options: light, dark, and device (follows system preference). The implementation handles theme persistence in settings, dynamic theme application, and extends dark mode styling to app customizer iframes.

Key Changes:

• Added theme setting with three modes: light, dark, and device
• Implemented theme application logic that respects user preferences and system settings
• Applied dark mode styling to app customizer iframes

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[Copilot]

The repaintSite function is defined but never called anywhere in the code. This function should either be removed or integrated into the theme application logic if it's needed.

Suggested change

	function repaintSite(){
	//tricks browser into repainting the site to apply theme changes
	const body = document.body;
	body.style.display = 'none';
	body.offsetHeight;
	body.style.display = '';
	}

[Copilot]

These blank lines appear to be unnecessary and should be removed to maintain code consistency.

Suggested change

[Copilot]

The iframe styling always checks the system's color scheme preference using matchMedia, but it should respect the user's theme setting (SETTINGS.theme) instead. When the user selects "light" or "dark" explicitly, the iframe should use that theme rather than always following the system preference. Consider using the applyTheme logic or a similar approach to determine which theme to apply based on SETTINGS.theme.

Suggested change

	// Style custom apps for dark mode
	const iframeDoc = iframe.contentDocument || iframe.contentWindow.document;
	if (iframeDoc && iframeDoc.body) {
	const bgColor = window.matchMedia('(prefers-color-scheme: dark)').matches ? '#2c2c2c' : '#ffffff';
	const textColor = window.matchMedia('(prefers-color-scheme: dark)').matches ? '#FFFFFF' : '#000000';
	// Style custom apps according to theme (light/dark/device)
	const iframeDoc = iframe.contentDocument || iframe.contentWindow.document;
	if (iframeDoc && iframeDoc.body) {
	// Determine effective theme: use SETTINGS.theme if available, otherwise default
	const currentSettings = (typeof SETTINGS !== "undefined" && SETTINGS) ? SETTINGS : DEFAULTSETTINGS;
	const theme = currentSettings && currentSettings.theme ? currentSettings.theme : DEFAULTSETTINGS.theme;
	const prefersDark = window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches;
	const isDark = theme === "dark" || (theme === "device" && prefersDark);
	const bgColor = isDark ? "#2c2c2c" : "#ffffff";
	const textColor = isDark ? "#FFFFFF" : "#000000";