Complete redone of the theme switcher.
The function that ran every 24hrs is gone, instead the calculation is done every time.
Instead of doing weird and possibly fragile string manipulation, it now uses the function solar-sunrise-sunset.
Fixed some errors, including a let expression.
Fixed so that the theme won't be reapplied if it was the last theme to be applied.
Fixed a syntax error (oops). The code now also compiles without errors.
I've not tested if polar night or midnight sun works as intended.