When checked, the palette should change
from this to this
simulating the lighting on a dark map.

It would be ok i think if it does that regardless which palette is chosen, while ingame only unit*.pal have these glowing colors.
For that it just reduces the brightness of the currently selected palette by 50% for all colors from index 0 to 239, so the glowing colors 240-254 stay in normal brightness.

This would help getting the glowing colors better done on SHPs.

I know a separate palette could be created, though this option shouldn't be that difficult to implement.
Turning a checkbox on/off is also faster than loading a different palette, which would also only clutter up the palette folder.