Submenu in Blix?

Sorry if this have been asked before, I tried searching.

Take a look at https://timmi.pointclark.net/, see the menu with Hjem/Arkiv/om etc, what I would like to do is make a submenu under it.
When I write a new page, say a child of Musikk (music), the Music in main menu will be active and the child will become visible.
I don’t know if I’m clear about what i want ??
No Javascript, I want to do it in PHP if anything.