The screenshot is from the site with Joomla upgraded to 3.6.0.
After that I downgraded the site again to 3.5.1.
In 3.5.1 and the custom.css completed with the code you gave, the webpage is what you now see with the given link. (
https://handelingen.com/index.php/redactie)
So in Joomla 3.5.1 it works quite well.
Further more, I remember now that in 3.6.0 it didn't matter that I had changed the custom.css in the way you suggested, also with your given code.
So I think it's the upgrade to 3.6.0 that causes the problems as in the screenprint.