Add some custom CSS to your theme / template
In the file
You can add any rule and property into a custom.css file. This file is located into
TEMPLATENAME/css/custom.css
You can edit the directly via FTP if you want to work on it and see the result in your website, but you can also use the interface to edit it with an integrated editor.
In the interface
Go in the menu Parameters >> Custom CSS to open the editor
You will get a popup where you can write your own CSS