Custom CSS in emails

Custom CSS (an early access feature) gives you fine-grained control over your email styling beyond what the drag-and-drop design settings offer. You add it in your channel’s Settings → Sending emails, in the Email CSS section — it applies channel-wide, not per edition. If you don’t see the section, it may not be enabled for your account yet.

How it works

The Email CSS editor accepts standard CSS rules that apply across your emails:

.my-block {
  background-color: #f0f0f0;
  padding: 20px;
  border-radius: 8px;
}

Letterhead applies your styles in place of its defaults. As the app notes in the editor: Letterhead can’t provide troubleshooting support for custom CSS implementation issues — you own what you write here.

Limitations and best practices

• Email clients vary widely in CSS support — always send yourself a test email and check it in the clients your audience uses (Gmail, Outlook, Apple Mail, and so on).
• Avoid advanced layout features like CSS Grid or Flexbox unless you’ve tested compatibility.
• Keep media queries simple for responsive tweaks (e.g., @media (max-width: 600px)).
• Test colors, fonts, and spacing across clients before sending.

Related

For per-edition design control without CSS, use the composer’s design settings, and see How to Adjust Mobile Font Size and Padding for mobile-specific sizing.