Can't install themes.

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.

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