Next to the Content section, you will see Design. This allows you to customize all of the colors across your landing page.
On the far right of your screen you will see a phone preview. As you are making changes, you can look to the phone to see how your changes are rendering to see if you need to make additional edits.
You can click Advanced Editor to edit using Custom CSS if you prefer (Note - Custom CSS will override all other values in basic editor. This feature is only intended for customer with advanced CSS experience.)
After making any changes, be sure to click Save and reload to save your work!