markus1988 Hi - welcome to Metabullet :)
From viewing the link you sent me, I notice that the platform is in fact Discourse. Whilst Flarum has many similarities, some parts of the theme you've referenced won't be possible without hacking the UI somewhat - in most cases, CSS positioning will allow to to achieve most of what you are looking for, although other remaining elements will require modifications to the core via an extension.
I'm going to be working with @JAC on a theme for his site, although this isn't ready for release yet.