For some reason my UV background turned bluescreen-blue, which
a) is ugly
b) does not show overlapping uv faces (this is actually my problem)
I have no idea what happened or how to get back to the original. Bug or feature?
Grab the top menu bar and drag it down to reveal the preferences.
Click the “Themes” button along the bottom.
Click the middle button in the second column from the left (probably labeled “3D View” and select “UV/Image Editor” from the pop-up list.
The bottom button should automatically read “Background”. (If not, click it and select it from the list.)
Use the sliders in the 3rd column to adjust your bg color. The default is 53, 53, 53.
Don’t forget to press Ctrl-U to save your changes.
Thanks, but the problem is not the color itself, but rather the fact that I am in some mode that does not visualize the overlapping uv coordinates.
Funny: I somehow got out of that mode, made an LSCM recalculation, forgot to pin some islands - bang! my uv was messed. No undo. So I had to reload a previous version and I don’t know how get the standard uv mode. 
View >> Draw Faces.
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Undo => Ctrl_z
Yeah, that’s what I thought, but then everything is blue, when deselected, nothing is blue (of course). But no mode shows my overlaps.
I know what I write sounds stupid, but I’m kinda lost.
Undo does not work for LSCM unwrap (actually I think it only works for mesh editing operations).
If you select UVCalculation Mode as Standard, Bounds, From Window or Cube (dealing only with rectangular model here), then there will be faces overlapping. (if you hit C you will then be able to select whole faces in the UVEditor).
There is only one way to avoid this and that is to prepare your model with good seams and use LSCM. Even if you calculate with “From Window” and map first the front and then the back the faces facing up and down, left and right are squashed because you are viewing them side on.
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