Let the theme be forced onto mail that styles itself

Appearance gained "Apply the theme to messages too" some time ago, and it
themes an HTML message only when the message brings no colours of its own.
That predicate is the right default and it almost never passes: one
`color:#FFFFFF` on one button label opts a whole message out, so in real
mail — receipts, shipping notices, anything from a template — the switch
did nothing at all and the reader kept a bright white card on a dark UI.

A second switch, off by default and only meaningful with the first on,
forces the palette over the sender's colours. It cannot be done perfectly,
which is why it is a separate, explicit choice: the same bargain a
dark-reader extension makes.

What it does is tell two kinds of colour apart. A *sheet* the design sits
on — the white 600px wrapper — is neutralised, and a *painted surface* —
a call to action, a footer banner — is kept whole so its label stays
legible on it. Relative luminance decides, at 0.5: white wrappers sit at
1.0, a blue button near 0.09. Only the painted ones are marked, with
data-ihm-keep, and one rule in EMAIL_BASE_CSS neutralises everything else.

Nothing the sender wrote is removed, so the switch is reversible, colours
arriving from a <style> block are covered as well as inline ones, and
print still pins the tokens to ink on white.

The mock grew the message this is about: an outer wrapper on
bgcolor="#ffffff", a <style> block, a coloured button, a grey footer.
Without one, neither the bug nor the fix could be seen.

Verified in a browser against the mock: with only the first switch on the
card is still white; with both, the wrapper computes to transparent, body
text follows the theme, and the button keeps white-on-blue. Two surfaces
marked, which are the two the message paints.

Closes #290

web/src/store/settings.ts

@@ -130,6 +130,15 @@ export interface Settings {
   imagePolicy: ImagePolicy;
   /** Let messages follow the app's light/dark theme instead of always sitting on white. */
   themeMessageBody: boolean;
+  /**
+   * Extend that to mail which brings colours of its own.
+   *
+   * Only meaningful with `themeMessageBody` on. Off by default because it
+   * cannot be done perfectly: see `markKeptSurfaces` in lib/html.ts for the
+   * bargain it makes, and #290 for why the conservative default alone left
+   * essentially all HTML mail on a white card.
+   */
+  themeStyledMessages: boolean;
   undoSendSeconds: number;
   composeFormat: ComposeFormat;
   replyAllDefault: boolean;
@@ -307,6 +316,7 @@ export const DEFAULT_SETTINGS: Settings = {
   knownSigners: {},
   imagePolicy: "ask",
   themeMessageBody: false,
+  themeStyledMessages: false,
   undoSendSeconds: 8,
   composeFormat: "html",
   replyAllDefault: false,