It's definitively not something most user will want. Yes, they want to style buttons, headings and other parts of their sites, but they really don't want to use CSS-code to do it. If they're advanced enough to install a plugin, they should (like the ones mentioned or even Child Theme Configurator It would be enough and much better to have the possibility to add a class to an element and maybe have a customizer control to define color, background-color and the like for the class.