Before importing a new theme into the Portal, you need a valid theme. To be valid, a theme needs to contain a desc.json file. Please check this article for more information.
In order to import a new valid theme into the Portal, you need to access your Newired Portal server.
If you already have a theme ready to be imported, just copy the theme folder inside PORTAL FOLDER\themes
Then, access the Portal and click on the Add-ons button.
Scroll down until you find the Themes section, and click on the Reload button to refresh the themes present on the Portal.
If your theme is not valid, a warning box will show up, and the invalid theme will simply be ignored.
A successful import means that the theme is properly shown in list of themes after reload button is pressed.