I think basically, this theme customization is about you replace certain graphic elements (and adjust other parameters such as: size, margin, fonts, etc...) in a specified Microsoft file format (*.msstyles) with support of 3rd tool that built solely for this task like Windows Style Builder. But that's not all of that, usually for the completeness of a theme, you will need to replace graphic elements in other system dynamic link library (*.dll) all around system folder, some *.cpl too (which used other tool like Reshacker/Restorator/etc...).