Change MyPaint theme (Ubuntu)

I’m trying to change the theme MyPaint uses. No success so far.

I have been able to change it by changing the system theme to a bright theme that I don’t like. When I do that MyPaint gets a dark theme that’s ok.

The commands I’m trying are these, only difference is that I’m trying various theme paths:

GTK_THEME=/usr/share/themes/Radiance/gtk-3.0/gtk.css mypaint

GTK2_RC_FILES=~/.themes/A-New-Hope/gtk-2.0/gtkrc mypaint