This is the classic theme found in Audacity versions up to and inclusive 3.5. It is being retired from active duty, but still may be used as a custom theme.
To use this theme in Audacity, copy ImageCache.png into the Audacity preferences folder and select the Custom theme in the preferences. Details can be found here: https://support.audacityteam.org/basics/customizing-audacity/using-themes
As Audacity evolves, the ImageCache will need to be regenerated. Doing this requires knowledge on how to compile Audacity and modify a little bit of text here and there. Instructions can be found here: https://audacity.gitbook.io/dev/scripting/creating-custom-themes
These files have been taken from https://github.com/audacity/audacity. A history of the files can be found here.