Viewer: Change the default color scheme to "System" which matches the colors used by the rest of calibre/the operating system

Also means that if you switch color schemes while the viewer is running,
the viewer will follow along automatically.

src/calibre/gui2/viewer/web_view.py

@@ -269,6 +269,7 @@ class ViewerBridge(Bridge):
     background_image_changed = to_js()
     goto_frac = to_js()
     trigger_shortcut = to_js()
+    set_system_palette = to_js()
 
 
 def apply_font_settings(page_or_view):
@@ -384,6 +385,15 @@ class Inspector(QWidget):
         return QSize(600, 1200)
 
 
+def system_colors():
+    pal = QApplication.instance().palette()
+    return {
+        'background': pal.color(pal.Base).name(),
+        'foreground': pal.color(pal.Text).name(),
+        'link': pal.color(pal.Link).name(),
+    }
+
+
 class WebView(RestartingWebEngineView):
 
     cfi_changed = pyqtSignal(object)
@@ -417,6 +427,7 @@ class WebView(RestartingWebEngineView):
         self.dead_renderer_error_shown = False
         self.render_process_failed.connect(self.render_process_died)
         w = QApplication.instance().desktop().availableGeometry(self).width()
+        QApplication.instance().palette_changed.connect(self.palette_changed)
         self.show_home_page_on_ready = True
         self._size_hint = QSize(int(w/3), int(w/2))
         self._page = WebPage(self)
@@ -508,9 +519,15 @@ class WebView(RestartingWebEngineView):
     def on_bridge_ready(self):
         f = QApplication.instance().font()
         fi = QFontInfo(f)
+        ui_data = {
+            'all_font_families': QFontDatabase().families(),
+            'ui_font_family': f.family(),
+            'ui_font_sz': '{}px'.format(fi.pixelSize()),
+            'show_home_page_on_ready': self.show_home_page_on_ready,
+            'system_colors': system_colors(),
+        }
         self.bridge.create_view(
-            vprefs['session_data'], vprefs['local_storage'], QFontDatabase().families(), field_metadata.all_metadata(),
-            f.family(), '{}px'.format(fi.pixelSize()), self.show_home_page_on_ready)
+            vprefs['session_data'], vprefs['local_storage'], field_metadata.all_metadata(), ui_data)
         for func, args in iteritems(self.pending_bridge_ready_actions):
             getattr(self.bridge, func)(*args)
 
@@ -592,3 +609,6 @@ class WebView(RestartingWebEngineView):
 
     def trigger_shortcut(self, which):
         self.execute_when_ready('trigger_shortcut', which)
+
+    def palette_changed(self):
+        self.execute_when_ready('set_system_palette', system_colors())

src/pyj/read_book/globals.pyj

@@ -48,17 +48,28 @@ def set_toc_anchor_map(val):
     toc_anchor_map.value = val
 
 default_color_schemes = {
+    'system':{'foreground':'#000000', 'background':'#ffffff', 'name':_('System')},
     'white':{'foreground':'#000000', 'background':'#ffffff', 'name':_('White')},
     'black':{'foreground':'#ffffff', 'background':'#000000', 'link': '#4f81bd', 'name':_('Black')},
     'sepia-light':{'foreground':'#39322B', 'background':'#F6F3E9', 'name':_('Sepia light')},
     'sepia-dark': {'background':'#39322B', 'foreground':'#F6F3E9', 'link': '#4f81bd', 'name':_('Sepia dark')},
 }
 
+
+def set_system_colors(spec):
+    c = default_color_schemes.system
+    c.foreground = spec.foreground
+    c.background = spec.background
+    c.link = spec.link
+
 register_callback(def():
     # Ensure the color scheme names are translated
     for key in default_color_schemes:
         scheme = default_color_schemes[key]
         scheme.name = gt(scheme.name)
+    # set the system colors if in dark mode
+    if window.matchMedia and window.matchMedia('(prefers-color-scheme: dark)').matches:
+        set_system_colors({'background': '#111', 'foreground': '#ddd', 'link': '#6cb4ee'})
 )
 
 runtime = {

src/pyj/session.pyj

@@ -37,7 +37,7 @@ defaults = {
     'background_image': None,
     'background_image_style': 'scaled',
     'background_image_fade': 0,
-    'current_color_scheme': 'white',
+    'current_color_scheme': 'system',
     'user_color_schemes': {},
     'base_font_size': 16,
     'controls_help_shown_count': 0,

src/pyj/viewer-main.pyj

@@ -16,7 +16,7 @@ from modals import create_modal_container
 from qt import from_python, to_python
 from read_book.db import new_book
 from read_book.footnotes import main as footnotes_main
-from read_book.globals import runtime, ui_operations
+from read_book.globals import runtime, ui_operations, set_system_colors
 from read_book.iframe import main as iframe_main
 from read_book.shortcuts import add_standalone_viewer_shortcuts
 from read_book.view import View
@@ -214,19 +214,27 @@ def create_session_data(prefs, local_storage_data):
 
 
 @from_python
-def create_view(prefs, local_storage, all_font_families, field_metadata, ui_font_family, ui_font_sz, show_home_page_on_ready):
+def create_view(prefs, local_storage, field_metadata, ui_data):
     nonlocal view
-    runtime.all_font_families = all_font_families
+    set_system_colors(ui_data.system_colors)
+    runtime.all_font_families = ui_data.all_font_families
     library_data.field_metadata = field_metadata
-    document.documentElement.style.fontFamily = f'"{ui_font_family}", sans-serif'
-    document.documentElement.style.fontSize = ui_font_sz
+    document.documentElement.style.fontFamily = f'"{ui_data.ui_font_family}", sans-serif'
+    document.documentElement.style.fontSize = ui_data.ui_font_sz
     if view is None:
         create_session_data(prefs, local_storage)
         view = View(document.getElementById('view'))
-        if show_home_page_on_ready:
+        if ui_data.show_home_page_on_ready:
             view.overlay.open_book(False)
 
 
+@from_python
+def set_system_palette(system_colors):
+    set_system_colors(system_colors)
+    if view:
+        view.update_color_scheme()
+
+
 @from_python
 def show_home_page():
     view.overlay.open_book(False)