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/locales/zh-Hans.ts

@@ -517,6 +517,8 @@ export const catalog: Catalog = {
     "Show labels in the sidebar": "在侧边栏中显示标签",
     "Collapse sidebar to icons": "将侧边栏收起为图标",
     "Apply the theme to messages too": "邮件也应用主题",
+    "Apply it even to mail that styles itself": "即使邮件自带配色也套用",
+    "Most marketing and receipt mail sets a colour somewhere, so the setting above leaves nearly all of it on a white card. With this on, the theme is forced over the sender's own colours: backgrounds they laid the message on are dropped, while buttons and coloured banners are kept so their text stays readable. Some mail will not survive it intact, which is why it is separate.": "几乎所有营销邮件和收据邮件都会在某处设置颜色，因此上面的设置会让它们几乎全部停留在白色卡片上。启用此项后，主题会覆盖发件人的配色：邮件所依托的背景会被去掉，而按钮和彩色横幅会保留下来，使其文字仍然清晰可读。有些邮件无法完好呈现，因此这是一项单独的设置。",
     "Swiping": "滑动手势",
     "Swipe left": "向左滑动",
     "Swipe right": "向右滑动",