The new themes have the Site editor, located in Dashboard: Appearance -> Editor
You can customize the Page Templates and individual Pages and Items from there. You can import and export Pattern.
Other themes instead have the traditional way you find in the Dashboard
You can add those from your Dashboard, by selecting Appearance > Customize, and then “Additional CSS“.
In that box you have to write in a markup language that is connected with the HTML5, this is called CSS3
Here you can find a starting point: https://en.wikipedia.org/wiki/CSS#CSS_3