Set default white background for all pages.

- fixes issue with dark themes

closes #602

src/lib/app/mainapplication.cpp

@@ -809,7 +809,11 @@ void MainApplication::reloadUserStyleSheet()
 
 QUrl MainApplication::userStyleSheet(const QString &filePath) const
 {
-    QString userStyle = AdBlockManager::instance()->elementHidingRules() + "{ display:none !important;}";
+    // Set default white background for all sites
+    // Fixes issue with dark themes when sites don't set background
+
+    QString userStyle = "body{background:white;}";
+    userStyle += AdBlockManager::instance()->elementHidingRules() + "{ display:none !important;}";
 
     QFile file(filePath);
     if (!filePath.isEmpty() && file.open(QFile::ReadOnly)) {