follow the instructions here:
then decky loader gives you loads of plugins to install - I’ve got the CSS plugin which lets you download custom themes, vibrant deck (really handy), steamdb art (which lets you change the artwork), and another thing that gives more accurate linux compatibility ratings.
oh, and another that lets me update all my flatpaks (heroic launcher, emulators etc) in gaming mode, which is probably my favourite. hate having to go into desktop to do that.