Refactor category styles to use CSS variables

[BeksOmega]

Basically the same steps as #8274.

See the design doc for more info.

This change should be marked as breaking.

[Deepakchowdavarapu]

@BeksOmega I would like to work on this issue

[BeksOmega]

Sorry @Deepakchowdavarapu This relies on some tricky internals of the library so it's not open for contribution ATM :/