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.