Updated from Sequoia to Tahoe. In settings I selected to show a colour on the menu bar (blue).
So when I open a picture by double click in Nitro the menu bar remains blue. When I click the green button nothing much happens in size but the menu bar turns black with white letters. Clicking the green button again the menu bar returns to blue.
It does not really bother me, but just curious to know what is going on.
The green button puts you into Full Screen mode. That’s an existing behavior of macOS – they may have changed the look a bit with the menus, but I don’t the functionality has changed.
If you want the zoom the window without going into Full Screen, double-click the title bar.