What is a good starting point to try several layouts? I have currently a nearly naked Flarum and as a first step I want some nice design.
Starting point for themes
You can use the custom header and CSS within the Admin Panel to manipulate Flarum to your liking.
I'd advise heading to the showcase tag to get some examples of what other members of the community have done with their own forums 
porschberg also, feel free to check out the Resources > Styles tag for themes others have created through CSS. Those can be a good base to build on.
porschberg I followed different approach and override some of the Flarum's default less variables and added new ones so it led me a comprehensive theme package. You can fork & broke it: https://discuss.flarum.org/d/22694-stargazing-theme
a year later
i think the best solution is to try some minor changes like this:
https://forum.justoverclock.it/
with custom css, custom header and custom footer.
Justoverclock omg what a huge header.
tolgaaaltas JustoverclockI omg what a huge header.
yeah i try to make it better, but i have no time this week 
Justoverclock I think you should use welcome banner as a header content. That would make more sense.
tolgaaaltas I think you should use welcome banner as a header content. That would make more sense.
don't know how could i do this XD