There isn’t really a way to save those changes of DevTools, or I am unaware of a way, to directly save those customizations. You can see DevTools more as an experimentation lab. The thing is, the CSS you edit there may or may not reflect directly the classes defined in the theme files, but most likely not exactly. So the way to do that is to create a theme, by cloning an existing one. Then you modify it slowly, and see if when applied (loaded), it looks like you expected. DevTools will be a very useful tool in the process, of course. But your changes should be done on the actual files of the theme and not by patching the already loaded theme (which will by definition exist only during that session).