Firefox Install using Firefox Add-ons Install the theme directly from Firefox Add-ons. Install Manually Download .xpi file or .zip file; In adressbar paste this about:addons; Go to Themes -> Settings -> Install Add-on from file; Select .xpi or .zip file; Apply Omni Theme. [Thunderbird] Install Manually Download .xpi file or .zip file; In the Thunderbird, go to Menu -> Add-ons -> Themes; Go To Settings -> Install Add-on from file; Select .xpi or .zip file; Apply Omni Theme.