feat(ui): add brand accent hook to GTK colour scheme

BRAND_ACCENT single source with shade()-derived states; accent CSS
covers suggested-action buttons and the listening chip, installed at
USER priority from main().

Refs #23

src/vocalinux/main.py

@@ -418,9 +418,10 @@ def main():
     if not check_display_available():
         sys.exit(1)
 
-    from .utils.gtk_color_scheme import apply_os_color_scheme
+    from .utils.gtk_color_scheme import apply_brand_accent, apply_os_color_scheme
 
     apply_os_color_scheme()
+    apply_brand_accent()
 
     if not check_appindicator_support():
         logger.warning("No StatusNotifierWatcher found on D-Bus session bus.")

src/vocalinux/utils/gtk_color_scheme.py

@@ -11,6 +11,12 @@ Portal ``Read`` is tried first. If it is unavailable or reports no preference,
 ``gsettings`` is used as a fallback. An unknown ``gsettings`` value or read
 failure leaves GTK defaults unchanged. ``GTK_THEME`` already set (AppImage or
 user override) skips the helper entirely.
+
+The Carbon-mapped house-teal accent hook lives here too: ``BRAND_ACCENT`` is
+the single source of truth (a later palette change is a one-line edit) and
+``apply_brand_accent()`` paints exactly recording state plus the primary
+action with it. Every other surface keeps OS theme variables, and overlay
+signal colours plus destructive reds are untouched.
 """
 
 import logging
@@ -32,6 +38,74 @@ _COLOR_SCHEME_KEY = "color-scheme"
 _PORTAL_TIMEOUT_MS = 1000
 _GSETTINGS_TIMEOUT_S = 1
 
+#: House teal from the Carbon-mapped palette (ticket #23). The single source
+#: of truth for the GTK accent hook: recording state and the primary action.
+#: White text on this opaque teal passes WCAG AA on light and dark themes.
+BRAND_ACCENT = "#0f6b57"
+
+#: Text colour used on top of the accent. Stays white across themes because
+#: the accent chip itself is opaque.
+_BRAND_ACCENT_ON = "#ffffff"
+
+
+def brand_accent_css() -> str:
+    """Return the accent-hook CSS, derived from ``BRAND_ACCENT`` only.
+
+    Covers exactly ``button.suggested-action`` (the primary action) and
+    ``.recognition-listening`` (the recording-state readout, restyled as an
+    opaque teal chip so contrast holds on light and dark theme backgrounds).
+    Hover/active states derive via ``shade()`` so no second literal creeps
+    in; no ``@theme_`` variable is referenced, so switching the system theme
+    needs no app restart.
+    """
+    return f"""\
+/* Voculinux house-teal accent hook (#23): recording state + primary action.
+   All other surfaces keep OS theme variables; overlay signal colours and
+   the reserved reds are untouched. */
+button.suggested-action {{
+    background-color: {BRAND_ACCENT};
+    color: {_BRAND_ACCENT_ON};
+    border-color: shade({BRAND_ACCENT}, 0.75);
+}}
+button.suggested-action:hover {{
+    background-color: shade({BRAND_ACCENT}, 0.85);
+    color: {_BRAND_ACCENT_ON};
+}}
+button.suggested-action:active {{
+    background-color: shade({BRAND_ACCENT}, 0.75);
+    color: {_BRAND_ACCENT_ON};
+}}
+.recognition-listening {{
+    background-color: {BRAND_ACCENT};
+    color: {_BRAND_ACCENT_ON};
+    border-radius: 6px;
+    padding: 2px 8px;
+}}
+"""
+
+
+def apply_brand_accent() -> None:
+    """Install the accent hook above per-dialog CSS without touching it.
+
+    Uses ``STYLE_PROVIDER_PRIORITY_USER`` so the hook wins over the
+    ``APPLICATION``-priority providers each dialog installs, while every
+    other rule in those stylesheets keeps following theme variables.
+    Fails soft: without a display the app keeps theme defaults.
+    """
+    try:
+        from gi.repository import Gdk, Gtk
+
+        css_provider = Gtk.CssProvider()
+        css_provider.load_from_data(brand_accent_css().encode())
+        Gtk.StyleContext.add_provider_for_screen(
+            Gdk.Screen.get_default(),
+            css_provider,
+            Gtk.STYLE_PROVIDER_PRIORITY_USER,
+        )
+        logger.debug("Applied brand accent %s", BRAND_ACCENT)
+    except Exception as exc:
+        logger.debug("Could not apply brand accent: %s", exc)
+
 
 def read_os_color_scheme() -> Optional[ColorScheme]:
     """Return ``prefer-dark``, ``prefer-light``, or ``None`` if unknown."""