Hello,
I would like to have an exactly 50 percent gray background in my theme (RGB 128,128,128).
I suppose I could edit an existing gtkrc file. But where to screw?
Tried to upload the 25… file, but new users can’t do that. 
Here it is: http://filebin.net/ft5c8wp5uv
Thx patdavid!
You can always host the file on something like filebin.net and link it here. Once you’ve spent a little time here on the forum you’ll eventually be able to upload stuff and add more links to posts (it’s an anti-spam measure).
I tried and found this (based on the 63-Gray-Cyan) and at least it is 50 percent gray 
May I ask why, i.e. for what purpose? In many cases, X% depends on Working profile, Output profile et cetera.
To have a neutral background for better orientation when developing an image: gray, because it is the middle of all colours, and 50% Gray, because it is the middle of black and white.
And the colour of the background should really not be influenced by the working or output profile of the opened image, that would be spooky…
At most by the display profile, but that is the idea of the display profile.