I love Audacity.
Straight-forward approach, simple & easy interface, but default themes are crap. Esp. at night.
High-contrast looks too alien. Dark theme looks too bright.
I do like the theme of Adobe Audition though. But I don’t like the complexity of the app.
There’s a theme for sale that looks like Audition made for Audacity, but I refuse, if I can make it myself.
So hereby 
How-to-install:
- Open Audacity, go to Preferences, Select Custom from theme menu. This way it creates a folder ‘themes’.
Win:
a. Open %appdata%\audacity\theme
b. Paste ImageCache.png there.
c. Click OK in Audacity (theme should now appear)
macOS:
a. Go to ~/Library/Application Support/audacity/Theme
b. Drop downloaded ImageCache.png there.
c. Click OK in Audacity (theme should now appear)
Linux:
a. Open ~/.audacity-data/Theme
b. Drop downloaded ImageCache.png there.
c. Click OK in Audacity (theme should now appear)
Special thanks to @mischamajskij for providing the decoded puzzle 
Thanks for your great work!
but there is still a small improvement, the selection line is black now and it’s very hard recongnize

i see you are using very dark red right now, it’s better to choose light red i guess
@ASDFGSWX
Weird, mine shows pink and I’m using my own template from above 

UPDATED THE THEME SO IT WORKS WITH AUDACITY 3.x!
Same instructions apply as in OP, but you need to put the .png and .htm file in a subfolder of Theme with the name custom, like this:

ImageCache.png + ImageCache.htm to download:
https://mega.nz/file/tIYSjIaS#jgMqKjIDCqH7iLmtWgywyxJnegizpZkXTM5E82QtCJg
Example of the theme on Windows (but should work fine on macOS and Linux too):

Saw some orange lines affecting the icons, so hereby the updated version of the link (hope moderator can copy paste this link to replace the previous one):
https://mega.nz/file/5QQAVY4Z#s-CHGzFgp2ajm8d26yZlfx29GLhGjJ0NdlxoBP_cs1s
Thank you so much for this! 
This theme is exactly for Audacity version 3.2.0 to 3.2.3
If you want to use it with another Audacity version, you can try to convert it using my converter.
https://github.com/vaclavmuller/AudacityThemeVersionConverter