Altium schematic editor colours
Altium prior to AD18, has a light design scheme.

You can try to change bits and pieces of the UI, but you will continue to fight it.

I'd recommend that you suck it up, stick to the defaults if you would rather be design-productive, and not fight the software at every step.

This may not be what you were hoping for, but after 20 years of Protel/Altium, you learn to pick your battles with this code.

Probable issues after setting every nook-and-cranny up just the way you like it: 


Printing issues  
Portability issues  
Future proof issues  
Update/upgrade issues (the option to import preferences during and update is a constant source of crashes)


As of yet, it doesn't necessarily get any better with AD18, which defaults to a dark theme, as it has many other issues (like: it is still beta class).

Sorry.  Don't fight it.  You'll thank me later.