I've used the "Custom CSS" option in Jetpack, as well as two standalone CSS plugins: Simple Custom CSS and SiteOrigin CSS. The SiteOrigin plugin is, by far, the best.

But in most cases, I back up my CSS manually to a text file that I save offline. That makes sense no matter what online CSS option you use,. It negates any issues you'll have if you switch themes. If the CSS changes don't port over to the new theme, then just copy in the ones you want from your saved file.

To the first-time WordPress user, this addition to the Customizer will make a lot of sense. CSS edits belong in the same place you're customizing other parts of your theme - and that's where I'd look for them if I weren't familiar with the WP dashboard. And they should have live previews.

Meanwhile, you can back up all your Customizer settings via a great free plugin made by the friendly folks at Beaver Builder. It's called "Customizer Import / Export" and it adds a backup option to the Customizer itself. It's easy for both experienced and novice users to understand and implement.