Alrighty. Spent way too much time on it, but here is pretty usable version of it.
Example:

ImageCache.png:

Replaced some icons with new ones from https://feathericons.com/, but their selection is small.
For documentations’ sake:
Colors that can’t be changed via theme:
- the left border next to “play/pin” icon toggle (white)
- the upper border for ruler (white)
- the monitor level (green)
And a few transparency issues which required workarounds:
- toolbar button backgrounds can’t have transparent background, or it will be filled with white
- button background image can’t be transparent where icon will come on top of it, or the background will be white
Attached is also (slightly messy) .xcf file for gimp if anybody wants to try out some improvements.
audacityxcfgimp.zip (71.1 KB)