I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this!
Issue:
Company Branding in Azure ADB2C - Can't edit or save.
Solution:
The issue was resolved by assigning the Global Admin role within the Azure B2C tenant.
Thank you again for your time and patience throughout this issue. If you have any other questions or are still running into more issues, please let me know.
Thanks,
Navya.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.