Add background css property to allow gradients

[Kunamatata]

Description

This PR allows using CSS gradients without touching any of the javascript files or extension code.
By adding background: var(--container-background-color); to style.css we enable users to do the following in their settings.json

Example:

{
  "codesnap.backgroundColor": "linear-gradient(62deg, #8EC5FC, #E0C3FC)"
}

[Kunamatata]

@kufii I've created my own fork of this project, but if you want to merge this in, this extension will have support for gradients.

An upgrade in the future might be to allow the user to set those colors in the web view panel.

[MinskLeo]

Also like this, would really appreciate if this will be added