Hi all!
I decided to make a flatter version of the light Soda Theme to better match OS X Yosemite. It’s not done yet, but I wanted to announce it in case anybody else was thinking about making one (feel free to fork and modify!) 
You can grab it at:
https://github.com/iccir/El-Capitan-Theme
Remaining work:
• Assets for Quick panel UI
• Assets for “Open Files” in the sidebar
• Tweaks for non-retina icons