Basically any upbeat song that reminds you to appreciate life in the middle of absorbing and applying theory. Anything from the Internationale to Red Army tunes to modern(ish) artists like Chumbawamba are allowed. Even Lofi.
Personal favorite of mine is Quand Fera-T-Il Jour, Camarade? by Mireille Mathieu. It's bittersweet, hopeful and resolute. Even if I don't speak French, I sometimes mumble to it.
Runner-up would be Solidarity Forever by Pete Seeger.
Commonwealth of Toil always helps. "But we have a glowing dream of how fair the world will seem, when each man can live his life secure and free... when the earth is owned by labor and there's joy and peace for all, in the commonwealth of toil that is to be"
Especially those first 6 words really just fill me with motivation and hope like nothing else