A style element would go in Custom Header
However, as far as best practices, you should not use a style element and instead put the contents of the element into your Custom CSS
I just wanted to point this out to whoever... not sure if there's a better way to report the problem.
Using the stock theme, if the title of the forum is too wide, the top bar on the page becomes twice as tall and the page title and breadcrumb get rendered beneath.
I mean where it says nodeBB in the top toolbar above. If it's something long like "My Awesome NodeBB Forum", that "My ..." text causes the top bar to become twice as tall and the top of the page (typically title and breadcrumb) get rendered beneath the top bar and are no longer visible.
It's even a problem with smaller titles, like "My Site" when you resize the browser smaller. The responsive design does work, but at a width bigger than a tablet/phone screen, the top bar will render as two lines.
My report isn't 100% correct.
When resizing the browser smaller, the top bar does become 2x taller, but the title and breadcrumb are rendered beneath properly.
So it's only a normal (not full screen) kind of browser window with a long site title that exhibits the problem.