I'm still struggling with GIMP. I haven't figured out how to change the color there yet, e.g. for buttom-active.
Edit: For the blue border I just downloaded the mx-matcha gtk theme from MX Linux repo on github (in addition to the matcha gtk theme by Vinceliuice, which I already had downloaded but doesn't have blue borders for the active window) and changed the style for the xfwm4 theme to mx-matcha.
But I´d like to learn how to set up any custom color.