Install any custom theme by placing the .ttheme in Tauon's theme folder

~/.var/app/com.github.taiko2k.tauonmb/data/TauonMusicBox (for flatpak)

~/.local/share/TauonMusicBox/theme/ (for aur)

Community theme thread: Taiko2k/Tauon#461

Wiki page on theming: https://github.com/Taiko2k/TauonMusicBox/wiki/Theming

Edited from this gruvbox theme: https://github.com/ggttttggggtt/tauon-gruvbox