Clean up CSS background color styles

Created by: sqs

Standardizes how components declare background colors. No components declare their own custom theme colors for backgrounds anymore; now they use CSS custom properties that can change based on the user's selected theme.

There will be minor changes to the background colors used in various components, but no (intentional) major changes. If we want to tweak colors further, we'll just need to introduce more variables that themes define (which is super easy).

Similar to #14847 and #14829.