1. Login to the wordpress multisite admin page
2. Hover on My Sites>>Network Admin>>Themes
3. Click "Add New" and then "Upload Theme"
4. Browse the Theme on your desk and then click "Install Now"
Note: In the Wordpress Multisite you will need to login as Network Admin in order to upload and install new theme as stated in step 2. Otherwise if you login just as normal user it won't work.
During the Installation there might be errors prompted like below
The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature
This is because the filder compression format is not compatible with the Wordpress (compression format such as .Tzar, .rar, etc are not compatible). The solution to this is to unzip the theme for example in this case i have to unzip the "AppThemes - Classipress v3.5.1 - Bestselling Classifieds Theme For WordPress.rar" and after that zip it back using the compression format *.zip
Then upload again and install and be working after that.
How to apply the theme after installation done.
1. Click on My Sites and new set of menu will appear
2. Click on Appearance>>Theme
3. You will see ClassiPress Theme on the list click "Activate" to apply it to the site
4. Save changes
5. Browse the URL of your web page the new ClassPress Theme should be working as expected