Theme Not in Dashboard Marketplace
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If it is a free theme, all you have to do, is go to the Marketplace in your browser, go to the themes page and scroll to the bottom. There should be a Download Archive button next to the version number. Click this and save it to your desktop. Unzip it and FTP it to the packages folder of your C5 installation.
Go to the Dashboard > Extend Concrete5. Scroll to the bottom of the page and look for the name of the addon. You should see a Install button to the right. Click this. The page should reload and a message should pop up near the top saying it has installed successfully.
You should then be able to enable it like usual.
-Blenderite
http://yourwebsite.com/index.php/dashboard/extend/themes/