I second @dklinger’s suggestion to use a separate monitor. If that's not possible, at least use an incognito window for quick testing on changes. This prevents super admin cross-contamination with authentication, menu visibility, and page access. Menu and page assignments could affect whether you see your CSS changes. Coming in as a potential user that you’re looking to target with edits will help make the testing more true to life.
The incognito window can serve as your live “learner” or user preview.
After saving any CSS changes, simply refresh the page. The changes will be almost instant for the “learner” user once saved in Branding Look and Feel by your other Super admin user.