Use proper variables for dark theme. (#3593)

packages/theme-dark-extension/style/variables.css

@@ -150,8 +150,8 @@ all of MD as it is not optimized for dense, information rich UIs.
   /* Defaults use Material Design specification */
   --jp-content-font-color0: rgba(255,255,255,1.0);
   --jp-content-font-color1: rgba(255,255,255,1.0);
-  --jp-content-font-color2: rgba(0,0,0,0.7);
-  --jp-content-font-color3: rgba(0,0,0,0.5);
+  --jp-content-font-color2: rgba(255,255,255,0.7);
+  --jp-content-font-color3: rgba(255,255,255,0.5);
 
   --jp-content-link-color: var(--md-blue-300);