No need to worry. You can experiment to your heart’s content with your browsers developer tools and won’t cause permanent damage to anything. (eg. right-click -> inspect, or F12)
IMHO, CSS is one of those things that’s relatively easy to grasp the basics, but gets more difficult the fancier you try to get with it (eg, gradients, svg, transitions etc.)
A main limitation is the HTML. If the mark-up is invalid or a mess in general, you can have problems no matter how good you are with CSS.
In other words, it might not be possible to move things around, but changing the font, color, spacing, hiding, etc. usually work OK without breaking something else once you determine the best selectors to use.