Brand-versioned assets

[bhallstein]

Ability for certain assets to auto-swap when the brand (theme) changes.

For example, if you're working on 3 potential brands (as custom themes), when changing between them you may need to swap assets (header images, background images, etc.) in line with the brand (in order for colours, styles, etc. to match each brand).

Possible implementation: individual media items could have option to have "branded versions" — with each version tied to a theme(/brand). Then using one of these brand-assets in the page editor would be the same as using a regular media item, but when changing theme it would automatically switch between branded versions.