Hello,
To change the font size and style of the header navigation Go to >> Theme Settings >> Typography >> Navigation font.
Screenshot for Clarification: https://www.screencast.com/t/rpTPyOHhS
It will only change the font size and styles of main header navigation. To change the typography of other elements in the header Go to >> Theme Settings >> Typography >> Advanced Typography >> Select the Module where you want to apply these changes.
Screenshot for Clarification: https://www.screencast.com/t/UMCkiBMPUYS
If it does not help then share your site WP Admin Login details in a private content area of this topic so that I’ll check it for you on your site.
Best Regards.
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