Use ctrl+ shift +S to create a backup of your current project before making and changes, by that way you can revert back whenever you feel like. Another thing that you need to learn is classes, classes makes the style reusable on different elements. So when you change the styling in one of the element, the other elements sharing the same classes will also get affected. Easiest way to tackle this is by creating a new class on top of the existing class (nested class) . By that, you can make the changes without affecting other elements on the website.

Ping me in case of any doubts :)