By skyforbes I used Open Bar to modify the shell and AdwSteamGtk flatpak to 1) add a GTK theme to Steam and 2) modify the .css file of one of the colorthemes so the result is this light mode (I'm still tinkering with it). The other apps were pretty easy to adjust.
(since this sub doesn't let me post multiple images, here's the most "egregious" one: Steam in light mode)