Consider this case:
I'm using a website service on a daily basis and I don't like the menu background-color. If I inspect the element I can see that the menu CSS class is called "my-menu" and with a developers tools I can alter the background-color, but obviously, when I'll refresh the page my change will be lost.
Is there a way to define that from now on whenever I surf to this website "my-menu" class will contain my changes? Is there a way to use CSS rules that was not defined in the loaded page and give them the top priority?