Add option to change the luminosity depending on the time of day

[NoschdarSindy]

This PR addresses #357

Beneath the luminosity slider I added a checkbox for dimming the Unsplash images at night time (see the image below). From 9PM on, if the box is checked, dark theme is used and the images are dimmed accordingly.

First-time contributor here, please let me know about any suggestions or if I missed anything