Most of the style settings of the building blocks can be configured in the app.
Sometimes, however, you may want to change style settings that are not configurable. This can be achieved using css. 

Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language like HTML.

Adding css requires technical skills.
To add custom css, find the css module in the settings of the app:

Add your css statements to the field and save: