Describe the problem

Sable Next's new Svelte UI has unique, readable CSS classes for nearly every element, so it will likely be far easier to theme in advanced ways than stable Sable.

Additionally, once Next replaces current Sable, certain themes that depend on old class names may no longer be completely functional.

Describe the solution you'd like

Backporting, or at least partially backporting, the CSS classes from Next where applicable and feasible would allow a transitionary period for theming.

I've done some work on putting in some CSS classes already, but I may not be the quickest at it.

Alternatives considered

we COULD just let themers cope about it when next comes out i guess???

Additional context

No response