Website editors can now add decorative SVG background shapes to snippets, including overlays on existing background images and customizable colors. The update also improves background controls so colors, images, and videos can be managed more independently, giving users more creative flexibility without overwriting existing choices.

This commit adds a new snippet option allowing the user to choose one of
a series of SVG shapes as their background, or to be overlaid over the
background image of a snippet.

this commit also introduces some utility functions to deal with multiple
css backgrounds.

task-2210790