Try to reset permalink if you don’t want to use polylang anymore WP admin > settings > permalinks and just hit save changes.
I don’t think that there is conflict with polylang, because I have used combination in one of my projects, and it is working without problems. Maybe you should try to deactivate other plugins like widget bundle etc. and to try again.
Regarding simple custom css plugin, it have to work without nay problems, so many users are using it, and not just in Moesia theme. In case that you want to remove it you can still add custom css by creating child theme https://codex.wordpress.org/Child_Themes and by adding css to child theme style.css file.
All the Best!