[Task]: Support CSS gradients in theme previews

[abyrne-moz]

Description

Firefox is looking to support gradients in themes, and these would need to be rendered correctly on AMO to show the gradient in the preview image.

https://bugzilla.mozilla.org/show_bug.cgi?id=2036647

Acceptance Criteria

• Theme previews show gradients when they are configured in the theme manifest.

┆Issue is synchronized with this Jira Task

[diox]

Blocked on the https://bugzilla.mozilla.org/show_bug.cgi?id=2036647 where the syntax is being finalized.

[diox]

Ironically we'll likely have to generate an intermediate SVG to generate the gradient for the preview...

[JulianaGaibler]

Oh, why's that?