Add StyleListener to ClientTheme to fix lightModeFixes

[CodeF53]

Light mode fixes of client theme replaces all --white-500 backgrounds with --primary-100, but since discord started lazy loading css chunks, the old approach of generating all these fixes at DOMContentLoaded is no longer functional.

This PR adds a mutationObserver to the head of the DOM to generate new fixes for lightMode as the app is used.

[Vendicated]

lint explosion