[docs] elaborate on theming for developers #1892
Description
Elaborate on the stub content in the file: put that stuff in the second half of the page, because it's more "under the hood" stuff
if the first section explain the "how to use" stuff:
- themable containers like surface, card, tile, accordion, etc
color-paletteattribute as the key to the whole system (which will not change when style queries land)- using
<rh-surface role="main">for whole-page theming - theming individual sections with surface