Dark Mode Simulation

simulateDarkMode

Generates a dark mode variant of the transformed HTML for a specific client.

function simulateDarkMode(
  html: string,
  clientId: string
): { html: string; warnings: CSSWarning[] }

Parameters

Parameter	Type	Description
`html`	`string`	Transformed HTML (output of `transformForClient`)
`clientId`	`string`	Client ID to simulate dark mode for

Returns

Field	Type	Description
`html`	`string`	HTML with dark mode CSS applied
`warnings`	`CSSWarning[]`	Dark-mode-specific warnings

Example

import { transformForClient, simulateDarkMode } from "@emailens/engine";

const transform = transformForClient(emailHtml, "gmail-web");
const darkMode = simulateDarkMode(transform.html, "gmail-web");

console.log(darkMode.warnings.length); // Dark-mode-specific issues

Dark mode support by client

Client	Dark mode	Behavior
Gmail Web	Yes	UI-only (no content inversion)
Gmail iOS	Yes	Full color inversion
Gmail Android	Yes	Partial color inversion
Outlook 365	Yes	Partial color inversion
Outlook (New)	Yes	Partial color inversion
Outlook Classic	Yes	Full color inversion
Apple Mail (all)	Yes	Respects `prefers-color-scheme`; partial inversion if absent
Yahoo Mail	Yes	UI-only (no content inversion)
Samsung Mail	Yes	Respects `prefers-color-scheme`; partial inversion if absent
Thunderbird	Yes	Respects `prefers-color-scheme` (v140+); full inversion if absent

For clients with UI-only dark mode (Gmail Web, Yahoo Mail), simulateDarkMode applies body wrapper styling but does not invert email content colors.

How simulation works

Emailens simulates dark mode by:

Inverting light backgrounds to dark equivalents
Adjusting text colors for contrast
Checking for prefers-color-scheme: dark media queries (clients that respect them skip forced inversion when present)
Applying client-specific dark mode behaviors (full vs partial inversion, UI-only darkening)

Note: Previews are simulated approximations rendered in Chromium. Actual rendering may differ in native email clients due to engine-specific quirks (e.g., Outlook's Word engine, Apple Mail's WebKit).