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)
Location of the themes options
Once you have chosen a theme, click OK to close the preferences dialog and load the new theme.
Installing custom themes
Custom themes commonly are distributed as files called ImageCache.png. You can find some custom themes here: