Viewer: When using the system color theme only override link colors in the book if the theme is dark. Fixes #1852990 [Viewer ignores color-property for links](https://bugs.launchpad.net/calibre/+bug/1852990)

src/calibre/gui2/viewer/web_view.py

@@ -387,12 +387,19 @@ class Inspector(QWidget):
 
 
 def system_colors():
-    pal = QApplication.instance().palette()
-    return {
+    app = QApplication.instance()
+    is_dark_theme = app.is_dark_theme
+    pal = app.palette()
+    ans = {
         'background': pal.color(pal.Base).name(),
         'foreground': pal.color(pal.Text).name(),
-        'link': pal.color(pal.Link).name(),
     }
+    if is_dark_theme:
+        # only override link colors for dark themes
+        # since if the book specifies its own link colors
+        # they will likely work well with light themes
+        ans['link'] = pal.color(pal.Link).name()
+    return ans
 
 
 class WebView(RestartingWebEngineView):

src/pyj/read_book/globals.pyj

@@ -60,6 +60,8 @@ def set_system_colors(spec):
     c = default_color_schemes.system
     c.foreground = spec.foreground
     c.background = spec.background
+    v'delete c.link'
+    if spec.link:
         c.link = spec.link
 
 register_callback(def():