Although I use sway, I used KDE for a long time and XFCE prior. They're both phenomenal. I'd love to see XFCE make its way to wayland in the future.
As an aside, I feel like Wayland has a market ripe for the introduction of lightweight DEs. Sure, it has the very lightweight (hyprland, sway, river, dwl) and heavyweight (KDE, Gnome) but nothing between like XFCE, LXDE or MATE
Kvantun for qt themes and lxappearance for gtk themes. In either one you'd have to first download a theme you want and apply it via correct program. You may have to export a shell variable to tell programs to actually use that theme (may not be necessary, I use a lot of flatpaks so for me it is)
The easiest way to get uniform colors in my experience is to full screen a program running the theme, use a dropper tool in Kolourpaint, see the hex color for what I want to change, then search for that hex in the theme file and change it to the one that matches my rice.