OSX EL Captain customization and themeing
Amazing post I wished to write a blog post about this same topic. For yosemite you had Flavour 2 for customizing. The was large of people contributing there themes. But, the support for Flavour 2 had ended. They won't support EL Captain because of SIP 

El Capitan Means The End Of Mac Themes &amp; Deep System Tweaks

But, there is still a work around for EL Captain and you can install Flavour 2. Have a look at this reddit post.