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, as shown in the image below.
Then, access the Portal and click on the Addons 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.