[[PR Description]]
- Adds doc comments for most fields on ThemeColors
- Scaffolds out some upcoming additional fields (Will add in a later PR)
- Renames a few fields:
    - `element_placeholder` -> `element_placeholder_text`
    - `element_drop_target` -> `drop_target_background`
- Removes the redundant `element_placeholder_text` (This should be set
using `text_placeholder`

Release Notes:

- N/A