Originally Posted by MasterPixelDestroyer
I'm trying to make a custom ROM (for now, only for x86, because I don't want to flash anything on my actual phone) and I want to modify the default UI throughout the entire system. This includes colors, outlines, corners on buttons, etc. Is there something in the AOSP source that would allow me to make these changes? I'm hoping I don't have to open the source for each individual application to change the look.
Sorry to say that but you'll have to mod each and everyone of your apps to have those changes (outlined, colors).
I've heard of a substratum theme that might fit your expectations, but I you want to do it yourself (which is more fun) you'll have to modify each and every apps. When you'll mod systemUI your modification will go "system wide" (quick settings and stuff) but the phone app won't be changed.
Have a good day