[Bug]: Alpha channel not supported in theme previews

[willdurand]

Is there an existing issue for this?

• I have searched the existing issues

What happened?

See https://addons.mozilla.org/en-US/firefox/addon/firefox-proton/, whose preview looks like this currently:

The black bar is not quite right.

What did you expect to happen?

That's because the manifest uses 4-number arrays for colors, the 4th one being the alpha channel. We should fix that.

┆Issue is synchronized with this Jira Task

[ioanarusiczki]

I've uploaded on dev the extension above and it looks good now https://addons-dev.allizom.org/en-US/firefox/addon/proton/