I use cmus as the music player on my laptop. I routinely use the track queue feature to create an immediate playlist of the next few songs, but never got to grips with creating persistent themed playlists. It turns out it’s not that hard, the documentation just seems a bit sparse.
Playlists are stored in plain text files, on my system inside ~/.config/cmus/playlists/. The default playlist (filename and playlist name) is created by default. It’s merely a list of filepaths referencing song locations, one filepath per line. The filename can contain spaces and so can the filepaths, with no quoting or escape sequences required. New playlists can be added by adding files to the directory. An example playlist:
/Volumes/share/music/Ben Caplan/Birds With Broken Wings/12 Ben Caplan - Canary.mp3
/Volumes/share/music/Bloc Party/Silent Alarm/07 This Modern Love.mp3
/Volumes/share/music/Family Atlantica/Cosmic Unity/06 La Humanidad.mp3
In cmus, it appears it is possible to add songs to the current playlist (y), marked by an asterisk in the playlist view (view 3). To select a playlist, highlight it and press :pl-create <playlist_name>. It’s also possible to create the playlists outside of cmus using a file manager, then load them automatically when cmus starts.