Email Clients
Thunderbird
Thunderbird rendering behavior and CSS support.
Overview
| ID | Category | Engine | Dark mode |
|---|---|---|---|
thunderbird | Desktop | Gecko | Yes |
Thunderbird uses Mozilla's Gecko engine (Firefox) and has excellent CSS support.
CSS support
Thunderbird supports nearly all modern CSS:
- Full flexbox and grid support
border-radius,box-shadow,opacity- CSS transforms, transitions, and animations
position: relative/absolutebackground-imagewith gradients- CSS filters
<style>blocks fully supported
Dark mode
Since version 140 (July 2025), Thunderbird has Dark Message Mode which applies dark mode transforms to email content. It respects @media (prefers-color-scheme: dark) — if your email includes dark mode styles, Thunderbird uses them. Otherwise, it applies full color inversion.
Add a prefers-color-scheme: dark media query for the best dark mode experience in Thunderbird.
Recommendations
Thunderbird rarely causes compatibility issues. Like Apple Mail, it supports nearly everything. Focus testing on Gmail and Outlook instead.