Hugh Every modern browser (desktop) has a way to Inspect elements on a webpage. Right click someplace, click Inspect (Element), and it'll open up the developer tools to give you a look at the code there. You may need to look at the surrounding code, too, depending on how something is built.
You can also change the code and play around, like a live-mockup, since your changes do not persist beyond a refresh.
After playing around with that, you can add changes to your Custom Header and Custom CSS boxes in the admin > appearance panel. If you encounter specific issues, I recommend a google search first, and then try pasting your code here to get some help. You'll also have to tell us what you're trying to accomplish if you get stuck.