This is probably “dynamic contrast” on your TV catching you out. This is typically enabled by default on modern TV’s and causes the TV to make adjustments to the brightness and contrast on the fly as the average brightness of the scene changes.
Personally I turn this and other such gimmickry off on my Samsung. The result of dynamic contrast can be that when the screen is largely a dark colour the few remaining brighter objects (such as the grey, unselected box) can get blown out to appear fully white and thus the grey and white can’t be distinguished. If you take a screenshot (with CTRL-S) of both the box being on and off and look at those screenshots on a PC you’ll see that one is white and one is grey even though they might both look the same on your TV.
Completely agree that the difference between the on and off states is too subtle though - one reason I don’t use the OSMC skin on my own systems.