Is there some (easy) way to make an hierarchical menu. I would like to add a number of pages and have these pages listed in a navigation menu.
But the number of pages will be too large to list them all listed in a “flat” menu, and additionally it would make sense to group them into subgroups. Unfortunately I haven’t seen a way to do this (and it has been a looong time since I messed around with CSS). Have I missed some way of doing this?
The perception of what is easy depends, as it often does, on who you are. 
Hugo, the blog engine that powers Micro.blog, has built-in support for nested menus. But it’s not something that Micro.blog exposes in the user interface, unfortunately.
So, while it’s trivial for a developer to implement a hierarchical menu via a custom theme on Micro.blog I wouldn’t say it’s easy for the majority of customers.
Suspected as much. I’ve played with Hugo a couple of times and also tried to create/modify a few themes (never had the patience to finish them). So I suppose it’s time to find a tutorial for hierarchical menus and clone an existing theme … should probably do this anyway.