Modifying the font
src/utils/typography.js if you want to make changes to the fonts.
Modifying the colors
src/gatsby-plugin-theme-ui/index.js if you want to adapt the colors, spacings and so on.
Customize your help center
Now that you know about modifying the theme, you can continue to learn how to customize your help center in any of these articles:
Or you can go straight to Deploying the Help Center.