> How the menu looks shouldn't be up to each individual player

Customisable UI is a highly debated topic amongst gamers and game designers; so why shouldn't it be up to the player? It comes down to personal preference and what individual people can perceive with a glance (which is different from person to person).
This is most definitely not a fully moddable UI that MMORPGs tend to use. I kept the options to a small number of easily distinguishable styles. Especially something like 'Menu Transparency' is a common option (citation possible if needed).
Why would say that e.g. Font Size is a valid customisation (or maybe no customisation at all?) as opposed to this? It also affects the visuals of the menu (quite substantially in OC because the menus scale with the size).

> it shouldn't be customizable ingame but in the game options

That is a problem indeed but a problem of how Clonk always managed options. The menus should most definitely not be customisable from a place where you can't immediately test the looks. Having an options page with UI options and only be able to see the difference after you've started a scenario would be horrible. And ingame, you can't access the options again and have to stop playing. This is a problem with many more options (like volume controls). To change this, a revision of the options in the engine is necessary in general. Also additional script interfaces, since the ingame menus are all done via script.