Bring Zed1 themes closer to their old appearance using the `base_theme`
values.

This will inevitably re-introduce some outstanding design issues from
Zed 1 that we will need to solve (low contrasts in some places, etc.)
We'll work on some of these in a later PR.

Known issues introduced:
- Scrollbar thumb is incorrectly drawn in front of the indicators
- Borders aren't quite correct - will fix in a followup
- Titlebar/status bar buttons will now have low contrast states
- Keybinding background color blends with palette

There are likely more issues, add them to the product checklist as you
find them.

Release Notes:

- N/A