Icecat's behavior is really weird: When you install a \*.xpi theme, Icecat will check the key `id`. But themes don't have that property. If you add that and compile from source, it works without any issues.  But still, do this will output some warning: ``` 1557641899973 addons.webextension.<unknown> WARN Loading extension 'null': Reading manifest: Error processing id: An unexpected property was found in the WebExtension manifest. 1557641900095 addons.webextension.<unknown> WARN Loading extension 'null': Reading manifest: Error processing id: An unexpected property was found in the WebExtension manifest. 1557641900111 addons.webextension.<unknown> WARN Loading extension 'null': Reading manifest: Error processing id: An unexpected property was found in the WebExtension manifest. 1557641900439 addons.xpi WARN Please specify whether you want browser_style or not in your options_ui options. 1557641900537 addons.xpi WARN Please specify whether you want browser_style or not in your options_ui options. 1557641900544 addons.webextension.<unknown> WARN Loading extension 'null': Reading manifest: Error processing id: An unexpected property was found in the WebExtension manifest. 1557641900572 addons.webextension.<unknown> WARN Loading extension 'null': Reading manifest: Error processing id: An unexpected property was found in the WebExtension manifest. ``` Close it and reopen, this will not show again.