0 Note normal sky colour.
1 Place a part-filled chest [under a glass roof] so that you can still see the sky in the background while the chest inventory view is open.
2 Hover over an empty slot in the chest, then move the mouse to hover over a slot with items in it.
3 Sky color changes from darkened to extra-darkened as you swap from a full to an empty slot and back again.

This is a pretty harmless glitch, but I don't see why where you hover the mouse should affect the sky color at all. The glass ceiling part may be optional, but that's the setup under which I see this problem. Screengrabs can be made available if needed.

Update: description clarified. There's three states; normal sky, darkened sky (clearly an intentional background fading as part of the UI), and extra-dark.