fix(ui): the message pane follows the desktop theme

The stylesheet hardcoded #bbb, #555, #000, #666, #ddd and #4a6f8a, and
set no background at all, so on a dark desktop plain-text mail rendered
as black on white inside a dark window and the web view's own default
showed through.

The colours now come from a Palette struct derived from QPalette, and
are passed into the builder rather than read from qApp inside it, so
the stylesheet can be tested against a known palette with no running
application. Base and Text rather than Window and WindowText: the pane
is a content surface like a text edit, and on many themes those differ.

The secondary colours are blends of text and background, not fixed
greys. That is the part that makes it work both ways round, since a
#555 chosen to read as subtle on white is nearly invisible on #2b2b2b.
The quote colour keeps its hue, because "this is quoted" is carried by
being a different colour rather than a dimmer one, but is pulled toward
the background so it stays readable instead of glowing on dark.

A sender's own HTML is deliberately left alone, and a test asserts that
so it cannot drift: rewriting a sender's styling would break layouts
that depend on it, and a newsletter setting a white background is
entitled to stay white. This themes the plain-text render and the
chrome around messages, nothing more.

MessageView passes its own widget palette rather than the
application's, since a style sheet or a themed parent can give the pane
different colours from qApp, and re-renders on PaletteChange: the
document's colours are baked into its stylesheet at build time, so
unlike a widget it does not restyle itself when the desktop theme
changes.

The load-bearing test asserts the negative, that no hex colour appears
in the style block which the palette did not supply. A test checking
only that the palette's colours are present passes with a leftover
literal still there, and one leftover literal is the whole defect.
Confirmed by mutation.

Closes item 12.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

4 files changed, 153 insertions, 14 deletions

diff --git a/src/htmlbuilder.cpp b/src/htmlbuilder.cpp
index ccb9171..c8f039f 100644
--- a/src/htmlbuilder.cpp
+++ b/src/htmlbuilder.cpp
@@ -18,25 +18,92 @@
 
 #include "htmlbuilder.h"
 
+#include <QCoreApplication>
+#include <QGuiApplication>
 #include <QRegularExpression>
 
 namespace {
 
-const char *kStyle = R"CSS(
-body { font-family: sans-serif; font-size: 10pt; margin: 12px; }
+/// The stylesheet, with every colour supplied by the caller.
+///
+/// %1 background, %2 text, %3 quote, %4 border, %5 dim.
+const char *kStyleTemplate = R"CSS(
+body { font-family: sans-serif; font-size: 10pt; margin: 12px;
+       background: %1; color: %2; }
 pre.plain { white-space: pre-wrap; word-wrap: break-word;
             font-family: monospace; margin: 0; }
-span.quote { color: #4a6f8a; }
-.message { border-top: 1px solid #bbb; padding: 10px 0; }
+span.quote { color: %3; }
+.message { border-top: 1px solid %4; padding: 10px 0; }
 .message:first-child { border-top: none; }
-.msg-header { font-size: 9pt; color: #555; margin-bottom: 8px; }
-.msg-header .who { font-weight: bold; color: #000; }
-.stub { font-size: 9pt; color: #666; padding: 4px 0;
-        border-top: 1px solid #ddd; }
+.msg-header { font-size: 9pt; color: %5; margin-bottom: 8px; }
+.msg-header .who { font-weight: bold; color: %2; }
+.stub { font-size: 9pt; color: %5; padding: 4px 0;
+        border-top: 1px solid %4; }
 )CSS";
 
+/// Mixes two colours, `weight` being how much of `a` survives.
+///
+/// Blending is what makes the derived colours theme-correct. A fixed grey is
+/// only "subtle" against the background it was chosen for: #555 reads as a
+/// quiet label on white and nearly vanishes on near-black.
+QColor blend(const QColor &a, const QColor &b, qreal weight)
+{
+    const qreal inverse = 1.0 - weight;
+    return QColor::fromRgbF(a.redF() * weight + b.redF() * inverse,
+                            a.greenF() * weight + b.greenF() * inverse,
+                            a.blueF() * weight + b.blueF() * inverse);
+}
+
 } // namespace
 
+HtmlBuilder::Palette HtmlBuilder::paletteFrom(const QPalette &palette)
+{
+    // Base and Text, not Window and WindowText: the pane is a content surface
+    // like a text edit, and on many themes Base differs from Window.
+    const QColor background = palette.color(QPalette::Base);
+    const QColor text = palette.color(QPalette::Text);
+
+    Palette result;
+    result.background = background;
+    result.text = text;
+
+    // Both derived from the pair, so they land at the right contrast whichever
+    // way round the theme is.
+    result.dim = blend(text, background, 0.6);
+    result.border = blend(text, background, 0.25);
+
+    // The quote colour keeps its hue, since "this is quoted" is carried by the
+    // colour being different rather than by it being dimmer, but it is pulled
+    // toward the background so it stays readable on a dark theme instead of
+    // glowing.
+    const QColor quoteHue(0x4a, 0x6f, 0x8a);
+    result.quote = background.lightnessF() < 0.5
+        ? blend(quoteHue.lighter(160), background, 0.75)
+        : blend(quoteHue, background, 0.85);
+
+    return result;
+}
+
+HtmlBuilder::Palette HtmlBuilder::defaultPalette()
+{
+    if (const QGuiApplication *app =
+            qobject_cast<QGuiApplication *>(QCoreApplication::instance()))
+        return paletteFrom(app->palette());
+
+    // No GUI application: only reachable from a test that did not pass a
+    // palette. A default-constructed QPalette is light, which matches what
+    // this code did before it was themed at all.
+    return paletteFrom(QPalette());
+}
+
+QString HtmlBuilder::styleSheet(const Palette &palette)
+{
+    return QString::fromUtf8(kStyleTemplate)
+        .arg(palette.background.name(), palette.text.name(),
+             palette.quote.name(), palette.border.name(),
+             palette.dim.name());
+}
+
 QString HtmlBuilder::renderPlain(const QString &text)
 {
     QString out;
@@ -61,12 +128,12 @@ QString HtmlBuilder::renderPlain(const QString &text)
     return out;
 }
 
-QString HtmlBuilder::document(const QString &bodyHtml)
+QString HtmlBuilder::document(const QString &bodyHtml, const Palette &palette)
 {
     return QStringLiteral(
         "<!DOCTYPE html><html><head><meta charset=\"utf-8\">"
         "<style>%1</style></head><body>%2</body></html>")
-        .arg(QString::fromUtf8(kStyle), bodyHtml);
+        .arg(styleSheet(palette), bodyHtml);
 }
 
 QString HtmlBuilder::namespaceCids(const QString &html, const QString &prefix)
@@ -218,14 +285,26 @@ QString HtmlBuilder::renderStub(const ParsedMessage &message)
 
 QString HtmlBuilder::build(const ParsedMessage &message, Mode mode)
 {
+    return build(message, mode, defaultPalette());
+}
+
+QString HtmlBuilder::build(const ParsedMessage &message, Mode mode,
+                           const Palette &palette)
+{
     ThreadRenderItem item;
     item.message = message;
     item.expanded = true;
-    return document(renderBody(item, mode));
+    return document(renderBody(item, mode), palette);
 }
 
 QString HtmlBuilder::buildThread(const QList<ThreadRenderItem> &items, Mode mode)
 {
+    return buildThread(items, mode, defaultPalette());
+}
+
+QString HtmlBuilder::buildThread(const QList<ThreadRenderItem> &items, Mode mode,
+                                 const Palette &palette)
+{
     QString body;
 
     for (int i = 0; i < items.size(); ++i) {
@@ -246,5 +325,5 @@ QString HtmlBuilder::buildThread(const QList<ThreadRenderItem> &items, Mode mode
                  renderBody(item, mode));
     }
 
-    return document(body);
+    return document(body, palette);
 }
diff --git a/src/htmlbuilder.h b/src/htmlbuilder.h
index 6e306dd..75fd1f8 100644
--- a/src/htmlbuilder.h
+++ b/src/htmlbuilder.h
@@ -18,7 +18,9 @@
 
 #pragma once
 
+#include <QColor>
 #include <QList>
+#include <QPalette>
 
 #include "mimeparser.h"
 
@@ -60,11 +62,46 @@ public:
         ForcePlain,  ///< Always render the plain part, escaped.
     };
 
+    /// The colours the document's own stylesheet uses.
+    ///
+    /// Passed in rather than read from qApp inside the builder, so the CSS can
+    /// be tested against a known palette without a running application, and so
+    /// nothing here depends on widget state.
+    ///
+    /// **Scope.** These style the chrome around messages and the plain-text
+    /// render. A message that brings its own HTML brings its own colours, and
+    /// those are deliberately left alone: rewriting a sender's styling would
+    /// break layouts that depend on it, and a newsletter that sets a white
+    /// background is entitled to stay white.
+    struct Palette {
+        QColor background;  ///< The pane itself.
+        QColor text;        ///< Body text.
+        QColor dim;         ///< Headers and stubs: present but secondary.
+        QColor border;      ///< Rules between messages.
+        QColor quote;       ///< Quoted lines in plain text.
+    };
+
+    /// Derives the document palette from a widget palette.
+    ///
+    /// The dim and border colours are blends rather than fixed greys, which is
+    /// what makes this work on a dark theme: a hardcoded #555 that reads as
+    /// "subtle" on white is nearly invisible on near-black.
+    static Palette paletteFrom(const QPalette &palette);
+
+    /// The palette used when a caller supplies none: the running application's.
+    /// Falls back to a light default with no QApplication, which only happens
+    /// in a test that did not ask for a palette.
+    static Palette defaultPalette();
+
     /// Single message, used for the error card and for tests.
     static QString build(const ParsedMessage &message, Mode mode);
+    static QString build(const ParsedMessage &message, Mode mode,
+                         const Palette &palette);
 
     /// The whole thread, oldest first.
     static QString buildThread(const QList<ThreadRenderItem> &items, Mode mode);
+    static QString buildThread(const QList<ThreadRenderItem> &items, Mode mode,
+                               const Palette &palette);
 
     /// Rewrites cid: URLs in an HTML body to their namespaced form.
     static QString namespaceCids(const QString &html, const QString &prefix);
@@ -73,5 +110,6 @@ private:
     static QString renderPlain(const QString &text);
     static QString renderBody(const ThreadRenderItem &item, Mode mode);
     static QString renderStub(const ParsedMessage &message);
-    static QString document(const QString &bodyHtml);
+    static QString document(const QString &bodyHtml, const Palette &palette);
+    static QString styleSheet(const Palette &palette);
 };
diff --git a/src/messageview.cpp b/src/messageview.cpp
index d7167bf..10b21a9 100644
--- a/src/messageview.cpp
+++ b/src/messageview.cpp
@@ -378,12 +378,26 @@ void MessageView::showDetailsDialog()
     dialog.exec();
 }
 
+void MessageView::changeEvent(QEvent *event)
+{
+    QWidget::changeEvent(event);
+
+    // Only when there is something to re-render: rendering an empty item list
+    // would replace a deliberately blank pane with an empty document.
+    if (event->type() == QEvent::PaletteChange && !m_items.isEmpty())
+        render();
+}
+
 void MessageView::render()
 {
     const HtmlBuilder::Mode mode =
         m_preferHtml ? HtmlBuilder::PreferHtml : HtmlBuilder::ForcePlain;
 
-    setDocument(HtmlBuilder::buildThread(m_items, mode));
+    // This widget's palette, not the application's: a style sheet or a themed
+    // parent can give the pane different colours from qApp, and the document
+    // has to match the frame it sits in rather than the app default.
+    setDocument(HtmlBuilder::buildThread(m_items, mode,
+                                         HtmlBuilder::paletteFrom(palette())));
     rebuildAttachmentBar();
 
     // Blocking is discovered during load, so check shortly afterwards.
diff --git a/src/messageview.h b/src/messageview.h
index 1be17f8..c55f5c5 100644
--- a/src/messageview.h
+++ b/src/messageview.h
@@ -105,6 +105,14 @@ protected:
     /// rather than one widget.
     bool eventFilter(QObject *watched, QEvent *event) override;
 
+    /// Re-renders when the desktop theme changes.
+    ///
+    /// The document's colours are baked into its stylesheet at build time, so
+    /// unlike a widget it does not restyle itself: switching the desktop from
+    /// light to dark would otherwise leave the open thread on the old palette
+    /// until the next selection.
+    void changeEvent(QEvent *event) override;
+
 private:
     void render();
     void updateHeader();