Actually, it seems a bit problematic to get every parameter to cooperate
I use this setting in The Gimp:
Edit | Preferences | Icon Theme | Symbolic | Guess icon size from resolution
and then click Reset.
So I could find the icons and changed them for a better look.
The thing is, that even, if the icons are made bigger (eg. 72x72 px), they stay small in gimp. Gimp seems to fix them on a 24px-size.
So instead of, I changed the colors to green and red and now it’s clear to see, if the aspect ratio is locked or not.