First of all, many thanks for the new Light-UI - it helps much!
I don't use it only under daylight conditions, it depends even on a picture, how bright it is. With the Light-UI and setting the Background Grey Level up, it is much more pleasant to work.
But to change these settings individual for a picture it is less intuitiv to go every time to Preferences and chose the User Interface for a modification.
My simple idea and a big wish: Please make a placeable Icon for the Menubar which allows you to switch between Light & Dark with only one click. The arrow on the right side allows you to change the Background Grey Level with a simple fader like in the Preferences/User Interface. Isn't that much more user-friendly? : )