You’ve probably figured this out by now but KDE has all the theming and rainmeter style widgets available. I just installed most of my desktop aesthetics direct from the System Settings on KDE, it’s mostly Catppuccin Mocha. Had to search for the Github for it to find some cool basic wallpapers that matched the theme perfectly.
Steam has some weird issues and I had it set to launch in terminal so the progress and errors get posted in there. Then I could search the web with the error codes. I don’t know the command though.
For games try ProtonDB.com, and then sort by “Tweaks” or whatever to see what people did to make the game run correctly.