Minor change to link color in dark theme. (#3430)

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

@@ -82,7 +82,7 @@ all of MD as it is not optimized for dense, information rich UIs.
   --jp-content-font-color2: var(--md-grey-500);
   --jp-content-font-color3: var(--md-grey-700);
 
-  --jp-content-link-color: var(--md-blue-600);
+  --jp-content-link-color: var(--md-blue-400);
 
   --jp-ui-font-scale-factor: 1.2;
   --jp-ui-font-size0: calc(var(--jp-ui-font-size1)/var(--jp-ui-font-scale-factor));