Login/About modal - create guidelines around what background images could/should be used

[srambach]

Describe the feature We should create some guidelines around what background images can be used and how they should be formatted. E.g., @lboehling created a background image with transparency so that it works in both default and dark themes. And the current about modal image and the positioning are tied - we could do a slight redesign and make it more resilient to different images.

Note... this might belong in content or design but I parked the issue in core for now.

[stale[bot]]

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