We recently published a blog post talking about mojoPortal skins which you should take a look at, here: https://www.mojoportal.com/mojoskins-project-release-v1
The information you're quoting regarding the codeplex site is quite old, and so are any skins contained in the "extra-skins.zip" file. If you want to get these skins anyway, they can still be found here: https://github.com/i7MEDIA/mojoportal/releases/tag/v18.104.22.168 (click the "extra-skins.zip" link on that page to download them).
Just so you're aware, we're planning to make some newer, prettier skins for mojoPortal available soon through the mojoSkins project. This will include skins with unique content styles (as opposed to the current ones which are essentially just bootstrap) so I hope you keep an eye on it.
Hope this helps!