Over the last few days I tried hard to costumize the default theme of pelican and succeeded somewhat. But in the end the inflexibility and the numerous overwrites in the css templates made it a giant timesink.
Nothing is perfect though and I had to change the way the cosmo theme handles tables and I was no big fan of the overwritten link style in the style.css.
Additionally there were some problems with the references towards the iconset which came with pelican-bootstrap3.
But for now I am happy and I think moving was the right choice to save time in the future.