Note: never copy-paste the code you've found on the internet if you don't understand it. Code snippets may contain malicious code which may harm your users.
Once you've done that select '+ Add membership'.
Select 'Settings' Tab.
Scroll down the page and locate 'Custom HTML' section:
Paste your code/styles into the text area and make sure the switch on the right is enabled for this feature to work.
Finally select 'Next' to complete and save your changes.
For more information on how to use Custom HTML for adding custom styles see this article.