Group/window background colour

I find the use of three different background colours in group/window titles rather confusing. I believe that a brown group contains links I didn't opened yet (?) and if I open it, it will become gray, but it's a concept/mechanism I don't need or really understand. (I use a part of my tree for bookmarks, this distinguishment is not needed there at all.)

A simple solution would be maybe if I could set the colours, but under the Colour settings I only have 4 options, these background colours are not available to change, it would be nice if that colour tab contained all UI elements in the tree.

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