You can customize your Substack by selecting different colors for your publication background, selecting from a variety of fonts, and also changing the publication layout.
When logged into your Substack account, head to your publication's Settings tab and click on Website in the left navigation bar. Then click on "Customize" next to Site Design.
Choose a layout, color, and fonts for your publication. Select “Set theme” to save your changes.
Tip: If you'd like to try random colors and fonts, click on the shuffle icon next to Edit theme on the Theme page.