Apple is using transparency in Safari controls (again). Recent Safari uses the webpage color. Is that what you’re encountering? That’s controllable by a Safari setting. An advanced setting, never use the background color in the toolbar, or some such.
If not…. Boot into Safe Mode, and try again. How to use safe mode on your Mac - Apple Support
If you have an add-on VPN client, an add-on anti-malware package, add-on cleaner apps, add-on ad-blockers,or other such apps, remove those and try again.
Private browsing records less info on the local system, but does not otherwise alter what remote apps can detect.’