Make seasonal background picture configurable

[tjwelde]

Description
The currently added seasonal background picture is visible as a blue background and can't be removed:

Proposal
Either don't add a background image to embedded cowswap widgets, or make it configurable

Alternatives
Since the widget renders an iframe, that runs on a different domain, than our site, there is not much we can do on our side.

[elena-zh]

Hey @tjwelde , thank you for bringing up this issue. It has been fixed. Please check it from your side.

[tjwelde]

Looks good, thanks