Hi Ghost73!
There was a bit of a dilemma with tiles mode on this issue. We could've kept the discovered tiles brighter and more obvious like they are in ASCII mode if tileset mode also used dots (which was the original intention), but the final decision was to go with full-on squares instead. The problem there is that even discovered squares must be really dark or they interfere with the foreground too much due to their greater pixel area (even slight increases in brightness change the feel of the map quite a lot, and can make other parts harder to read).
However, Kacper did follow the tileset color gradient quite strictly, where in this case it might not be necessary and there could be some room to tweak the brightness of those up a little without harming the overall aesthetic and readability of other elements.
Another part of the problem is that on some monitors the floor tiles will show up better due to how very dark colors are treated, while on others they'll remain fairly dark. In finding the right brightness these two situations need to be balanced somewhat (with regard to the default appearance).
That said, there could be some sort of gamma correction specifically for this issue... In fact, that sounds like a pretty good idea! The current default setting would be the darkest, and you could shift the floors to appear brighter if need be. I'll explore this possibility, though testing shows that if the floor is too bright, debris doesn't show up very nicely.
See some attached samples of the current base value compared to +50% and +100% brightness on the floor.