Click the settings button from the top left of the checklist

Select the FONTS & COLORS from side menu then you need to choose any color, and set according to your checklist's theme. 
Color includes: 
- Heading & Subheading of Checklist.
- Checklist Background
- Checklist Text.
- Checklist Textbox & Tick
- Opt-in form Text, Button, Loading bar, etc.
- Loading bar
- Footer