I built a site with the theme Corporate Clean. I wanted to add some custom CSS (override some colors, fonts etc.) so I added this to the files in the folder /sites/all/themes/corporateclean/...). Now an update for the theme became available and I lost all my custom styles (I have them in source control so no panic).
What is the best way to add custom css that doesn't disappear when a new theme version is published? I prefer file based. I've seen CSS Injector but that CSS would also be gone if I start from a new database.