I have edited the theme's:
I changed one class and it changed on the website. I have added another change and it did not reflect on the website.
How to refresh ossn.default.css?
All I know is that the 9 rules have always been a mystery. Some have stuck to them and it brought them luck. Others who followed regretted it bitterly. Unfortunately, you cannot foresee fate. It's probably best to simply forget about the 9 rules and make regular backups instead.
There are no special "saved CSS settings" when Ossn cache is disabled. When the site is loading all css files (theme and component parts) are glued to one file named ossn.default.css
Again use your browser console (network tab this time) to watch it being loaded. You may look into it from the console, too to find the changes you made. And as I said: there's always a chance that css doesn't work as expected when you're addressing an element some way wrong.
First of all: While developing DISABLE Ossn cache.
Then a brower refresh should show any change immediately.
But I think you already did it because otherwise you wouldn't notice your other change.
Actually CSS can be tricky sometimes. I'd recommend to use your browser's development console (right click page and choose 'Inspect') in order to try out your changes in place, first. This way you're getting a real-time result or at least an idea what isn't working as expected.