Goto Redmine's root folder
Copy and unarchive theme
cp circle_theme-3_0_0.zip /var/www/redmine/public/themes
This would result in a directory-path to application.css like:
Restart Redmine app
You now may need to restart Redmine so that it shows the newly installed theme in the list of available themes.
For Phussion Passenger just touch
Go to "Administration -> Settings" -> "Display" and select your newly created theme in the "Theme" drop-down list. Save your settings.