I had to do the same thing on one of my sites.
Firstly, you should create a Template Override for the mod_login module
Once done, you should have the following file:
Note: The path may be slightly different depending on the template's framework.
Then, Open the file mentioned above and add the following code
$doc = JFactory::getDocument();
You will of course need to change
#nav to your menu wrapped selector and
.item-222 a to the menu item selector.
If you provide a link to your site, I can give you the exact code used above, but until then, this is all I can provide.
Hope this helps