Are you looking in preview mode or on a published page? Sometimes certain customizations will only appear on a published page. Not sure if that’s the case here, but it might be worth double checking.

When you put the css in did you wrap it in a tag? 

To add to Joe’s point, I would suggest adding a new Stylesheet, rather than slotting it into a current one (just to avoid coding errors!). This will also make managing your CSS easier.

Select “Stylesheets” at the bottom > select “Add Stylesheet” at the top left. 

As Joe mentioned, open the coding with 
Paste your custom CSS
End the entry with   to close the code.