Improve CSS: Use Custom Properties for Flexible Animations

[fellyph]

Refactor to use CSS Custom Properties for animation ranges (e.g. ) to fix the broken 'Custom' range feature. Ensure these properties are properly injected via PHP and JS.

[github-actions]

🤖 Hi @fellyph, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.