Even when the Theme Navigation Menu is set through the builder, it does not appear in the page.
When inspecting the page, I've found that the container of the Theme Navigation Menu, the one with class
themeNav, has specified inside style
visibility: hidden. Since it is specified in the style, there is no way to override it with CSS.
There is a way to make the navigation menu visible in the Help Center standard template? I don't have this issue with other templates.