Hi Jeroen,
I have tried the code and it seems to work fine.
Perhaps there are some CSS errors in the Global CSS that is stopping the code from working. To verify that, please copy all the codes that are in the Global CSS and paste it here.
It should provide some information if there are any CSS error that is causing the issue that you have to fix so that the CSS codes will work.
Another option you can take is to use Cornerstone to build your blog post. To do that you just have to make sure that you enable the Posts in the Permissions Manager so that you can edit posts using Cornerstone.
https://i.imgur.com/juUfo0b.gif
Hope this helps.