Tori, to start making changes to your theme you need to first create a custom theme. 

1. To create your own custom theme, sign in to your control panel and click Websites > Theme > New custom theme. (Theming Video: nationbuilder.com/theme_documentation

2. Once you have a custom theme, you can then begin editing your stylesheet. To find it, go to Websites > Theme > theme.scss.

3. In your stylesheet, click the toggle button over from "Current" to "Default" to see your theme's original styles and find the one for the element you are trying to edit.

For example, your text body has the following styles: