doc: implement minor CSS improvements

* split selectors to one per line
* remove units from 0
* use double colon pseudo selectors consistently
* use `background-color` instead of the `background` shorthand since
  only the color is supposed to change
* remove leading zeros from numbers
* remove redundant values from shorthands
* use lowercase hex for colors
* use numeric values for `font-weight`

PR-URL: #29669
Reviewed-By: Benjamin Gruenbaum <[email protected]>