There are 2 ways to add custom CSS to Retool apps.
First, you can add any CSS you'd like to the "Preloaded CSS" area of the /settings/advanced page and that would affect all relevant css classes on all of your Retool apps.
Or, inside of your app you can click the three dots that give you a dropdown of App actions then head to scripts and styles. The CSS you write there will affect all components inside that app.
Here is a post breaking those two down further:
http://community.retool.com/t/how-to-write-custom-css-in-retool/2165