Ah, OK. You are not running the native Mac Mail app, but using Gmail from within your Google Chrome browser...
And, you are clicked out of Chrome. You are in the finder, which is why it says "Finder", and not Chrome next to the Apple.
You would get your Chrome menus at the top of the screen if you click within the mail window, or simply click on the Chrome app icon in the Dock.
What version of Chrome are you running? Current would be version 64 or something higher...
Or, maybe you have some extension, or setting that you are using in Chrome which affects the theme that you are using?
(How did you remove the tab line? I can't find any setting that will do that! Again, maybe an extension that you have installed)
You could try a reset within Chrome. Chrome/Preferences/Advanced/Reset.