Hi, it looks like a plugin is conflicting or possibly overriding parts of the theme.
The first thing to do is deactivate your active plugins one-at-a-time to see which plugin (if any) is causing the issue.
It is a process of elimination, so when we find the conflicting plugin we can then look into how to fix it.
Something is stopping the theme header from being rendered properly, and I can see a ‘Sumo’ login button in the header:
I am not familiar with this, is this powered by a WordPress plugin?