Instead of copying & pasting your theme code back and forth on the customize page, or even worse, editing it directly there, copy your CSS to an external style sheet, put it in your Dropbox public folder, get the public address, and link to it in your theme. Now you can edit the CSS, and your changes will be reflected automatically in your theme as soon as you save the file, since Dropbox will upload it automatically each time you save it. After you’re done and you’re happy with your changes, you can copy the CSS back into the theme, or save the external style sheet to your own server.