Audacity comes with 4 themes preinstalled: Light, Dark, Classic and High Contrast. You can switch between themes by going to Edit -> Preferences -> Interface (macOS: Audacity -> Preferences -> Interface)
Once you have chosen a theme, click OK to close the preferences dialog and load the new theme.
Custom themes commonly are distributed as files called ImageCache.png. You can find some custom themes here:
To install a custom theme, place the ImageCache.png in the following folders. You may create the folders if they aren't present.
Windows: C:\Users\<username>\AppData\Roaming\Audacity\Theme\custom\
MacOS: ~/Library/Application Support/audacity/Theme/custom/
Linux: ~/.local/share/audacity/Theme/custom/
Then,
restart Audacity,
open the Preferences -> Interface page,
select Custom as the theme and
press OK.
Be aware that custom themes may break from one Audacity version to the next.
