Is there a way to change the Facebook "blue" to a different color easily? on this theme: https://www.opensource-socialnetwork.org/component/view/2243/fake-book
You need to adjust colors manually (php) in:
While viewing your site, right click on the area you want to change and use your browsers "inspect" or "inspect element" feature, and it will give you an idea of which item you are looking for to make the change in default.php.
Now... as a better option (in my opinion), you could look at the "Green by Green" theme from ~Z~ Man. It has features other themes do not, a dark mode, and a BUILT IN color editor for the major areas you might want to modify. I have modified beyond that, but the editor it comes with does the vast bulk of any color changes you might want to make.
In my opinion, it is currently the best third party theme there is for OSSN.