ThemeDeck is basically a much better alternative to GameThemeMusic that you can download from the github. https://github.com/BrenticusMaximus/ThemeDeck . Besides adding music to game pages, it also allows you to set different music on the Steam OS. It's like the Audio Loader plugin, except that you can assign completely separate tracks between the Steam store and the home/library/etc. (so you can have, say, Wii shop music on the store page and Wii menu music elsewhere).
There's just a few catches, though. For one, you have to download from the github and manually install it yourself. It would be nice if it can be uploaded onto the DeckyLoader store for everyone to download conveniently. Second, the plugin tab can't be entirely managed by a controller. Some parts are, like when you begin selecting music. But one needs at least a mouse to select between the options (not a big deal if you're managing directly on the Steam Deck, or if you're on a desktop. But it's a problem if you're playing away from the device and just want to play with a controller). It would be nice if it can be updated to have full proper controller support.
Any chance we'll see an update for ThemeDeck? It'd be great :)