I got it working, here's how I did it:

1. Open devtools [F12]
2. Navigate to Sources, Overrides subtab, click [X] Enable Local Overrides
3. Navigate to Page subtab, find the stylesheet (usually in the css folder tree)
4. Right click the stylesheet and select "Save for overrides"

You can now edit in the main editor window and see changes live. CTRL-S to save.

To turn off local override, go back to Overrides sub tab and uncheck [ ] Enable Local Overrides