Hi,
I use the “Dark” theme in 2.9. One gripe I have with it is the background of the selected tool.
I would like to get it a bit darker and have a better contrast in with the icon. I looked into the theme and icon files but found nothing suitable.
Nice forum here! 
Rolf
I suspect it comes from your system theme. On my system (Manjaro / XFCE4) the background of selected tool is just a slightly lighter grey surrounded with a white border.
@rstein report the problem to GIMP https://bugzilla.gnome.org/ The bundled theme seems to be missing some values which get inherited from the system theme.
It’s good to see work being done by GIMP to bundle dark themes, but currently all of them have issues; they are not ready for production (unless you happen to be using a system theme whose colors happen to fit well with the GIMP themes when GIMP inherits them). @Carmelo_DrRaw for this reason I would suggest having the AppImage use the “System” theme + “Color” icon theme, until the problems are resolved.