Server and Viewer: Add support for dark themes in the UI

UI now uses dark colors when either the browser is in dark mode or the
viewer color theme is dark

src/pyj/book_list/theme.pyj

@@ -6,38 +6,46 @@
 LIGHT = '#F6F3E9'
 LIGHT_DARKER = '#b6b3a8'
 LIGHT_GRADIENT = 'linear-gradient(to bottom, {}, {})'.format(LIGHT, LIGHT_DARKER)
+DT_DARK = '#2d2d2d'
+DT_DARK_LIGHTER = '#777'
+DT_LIGHT = '#ddd'
+DARK_GRADIENT = 'linear-gradient(to bottom, {}, {})'.format(DT_DARK_LIGHTER, DT_DARK)
+
+def c(light, dark):
+    return {'light': light, 'dark': dark or light}
+
 
 DEFAULT_COLORS = {
     # General colors
-    'window-background': LIGHT,
-    'window-background2': LIGHT_DARKER,
-    'window-foreground': DARK,
-    'window-error-foreground': 'red',
-    'window-hover-foreground': 'red',
-    'link-foreground': 'blue',
+    'window-background': c(LIGHT, DT_DARK),
+    'window-background2': c(LIGHT_DARKER, DT_DARK_LIGHTER),
+    'window-foreground': c(DARK, DT_LIGHT),
+    'window-error-foreground': c('red'),
+    'window-hover-foreground': c('red'),
+    'link-foreground': c('blue', '#6cb4ee'),
 
     # Top bar specific colors
-    'bar-background': DARK,
-    'bar-foreground': LIGHT,
-    'bar-highlight': 'yellow',
-    'heart': '#B92111',
+    'bar-background': c(DARK, DT_DARK_LIGHTER),
+    'bar-foreground': c(LIGHT, DT_DARK),
+    'bar-highlight': c('yellow'),
+    'heart': c('#B92111'),
 
     # Item list colors
-    'list-hover-background': DARK,
-    'list-hover-foreground': LIGHT,
+    'list-hover-background': c(DARK, DT_DARK_LIGHTER),
+    'list-hover-foreground': c(LIGHT, DT_DARK),
 
     # Tree colors
-    'tree-highlight-item': LIGHT_DARKER,
+    'tree-highlight-item': c(LIGHT_DARKER, DT_DARK_LIGHTER),
 
     # Button colors
-    'button-start': DARK,
-    'button-end': '#49423B',
-    'button-text': LIGHT,
+    'button-start': c(DARK, DT_LIGHT),
+    'button-end': c('#49423B', '#666'),
+    'button-text': c(LIGHT, DT_DARK),
 
     # Dialog colors
-    'dialog-background': LIGHT,
-    'dialog-background-image': LIGHT_GRADIENT,
-    'dialog-foreground': DARK,
+    'dialog-background': c(LIGHT, DT_DARK),
+    'dialog-background-image': c(LIGHT_GRADIENT, DARK_GRADIENT),
+    'dialog-foreground': c(DARK, DT_LIGHT),
 }
 
 DEFAULT_SIZES = {
@@ -50,10 +58,21 @@
     'main': 'sans-serif'
 }
 
+
+def set_ui_colors(is_dark_theme):
+    attr = 'dark' if is_dark_theme else 'light'
+    s = document.documentElement.style
+    for k in DEFAULT_COLORS:
+        val = DEFAULT_COLORS[k][attr]
+        s.setProperty('--calibre-color-' + k, val)
+
+
 def css_for_variables():
+    is_dark_theme = window.matchMedia('(prefers-color-scheme: dark)').matches
+    attr = 'dark' if is_dark_theme else 'light'
     ans = v'[]'
     for k in DEFAULT_COLORS:
-        val = DEFAULT_COLORS[k]
+        val = DEFAULT_COLORS[k][attr]
         ans.push(f'--calibre-color-{k}: {val};')
     return ':root { ' + ans.join('\n') + '}'
 

src/pyj/read_book/view.pyj

@@ -8,7 +8,7 @@
 import read_book.iframe  # noqa
 from ajax import ajax_send
 from book_list.globals import get_session_data
-from book_list.theme import cached_color_to_rgba, get_color
+from book_list.theme import cached_color_to_rgba, get_color, set_ui_colors
 from dom import add_extra_css, build_rule, clear, set_css, svgicon, unique_id
 from iframe_comm import IframeWrapper
 from modals import error_dialog, warning_dialog
@@ -295,9 +295,10 @@ def __init__(self, container):
             ,
         }
         entry_point = None if runtime.is_standalone_viewer else 'read_book.iframe'
+        self.current_color_scheme = resolve_color_scheme()
         if runtime.is_standalone_viewer:
             document.documentElement.addEventListener('keydown', self.handle_keypress, {'passive': False})
-        self.current_color_scheme = resolve_color_scheme()
+            set_ui_colors(self.current_color_scheme.is_dark_theme)
         self.iframe_wrapper = IframeWrapper(
             handlers, document.getElementById(iframe_id), entry_point, _('Bootstrapping book reader...'),
             f'{runtime.FAKE_PROTOCOL}://{runtime.SANDBOX_HOST}/book/__index__')
@@ -718,6 +719,8 @@ def on_iframe_error(self, data):
 
     def apply_color_scheme(self):
         self.current_color_scheme = ans = resolve_color_scheme()
+        if runtime.is_standalone_viewer:
+            set_ui_colors(self.current_color_scheme.is_dark_theme)
         for which in 'left top right bottom'.split(' '):
             m = document.getElementById('book-{}-margin'.format(which))
             s = m.style