When switched to thumbnail, field colours remain visible like they were in power mode.
This might be easier to understand if we could see what it looked like in Power Mode.
To me it looks like there is a background image with a white background aligned to the bottom right of the file display, which doesn't fill the whole space, and the file display background colour has been set to light blue (when you'd want white to match the background image), which is being used to fill the remaining space.
You are absolutely right!
Blue colour comes from a filter background colour. What made me do a wrong assumption was that the blue area matched the size of first column and lines present in lister almost perfectly (purely by accident).
Sorry for disturbing.