Doesn't seem to work for normal menu items under the navbar, any other ideas, you can see that the menu items don't look as if they had the Noto Serif font.

Hi,

Just changing the font-family inside Theme options under Theme Menu inside Website Editorchanges the Mega menu and the child menus. 

In the above screenshot, the font in Open Sans.

In this screenshot, the font is Noto Serif.

Hope it helps