Dark theme for El Capitan?
I can suggest the following:


You could use the Freeware 'XtraFinder' (http://www.trankynam.com/xtrafinder). There is a workaround for OSX 10.11 to enable and it works great. I use it for File- and Folder Colors/Labels, a feature i missed very much and now i got it back ;-) 
The function you are looking for is "bright Text on dark background" for which you can define the colors to your liking for the window content. You cannot change the color of the window Frame, Title and Tabs, though.
You could use the commercial 'Pathfinder' (http://www.cocoatech.com/pathfinder/). It's often promoted as a Finder-Replacement and has tons of useful functions. One of them is hidden in the Preferences - the "Appearance"-Settings. Here you can configure nearly all display elements of PF. They offer a 30 Day Trial.


I hope one of these help you solve your Problem.