This video from Facebook/learnwordpress does a good job of showing how to edit your themes from within the WordPress (WP) Theme Editor. The narration is done by a Brit if it matters to you keenly. The author shows you how to change colors and dimensions along with margins.

While I truly can’t recommend fooling around with margins if you are just getting started with editing themes, the info is solid.

I’d suggest just sticking to the themes colors and image sizes to start. You can easily end up with a rather odd looking site if you set the margins wrong. The only margin I personally EVER edit is the top margin.

One last thing on the WP theme editor. I would really not advise leaving this on as it is a known WP security weakness.

You can edit themes on any computer by unzipping the file containing the theme. Just open the style.css file with your computers default text editor, (Wordpad for Windows) make your changes, and save. Then just close style.css and Zip the file back up and upload to WP after deleting the version you have installed on your blog to see your results.

You may also edit themes live with cPanel File Manager.