MediaWiki theme: Use distribution's image type for backgrounds
MediaWiki theme: Use distribution's image type for backgrounds

The checkboxes and radios would never look selected if one was using
the raster distribution.

Change-Id: I8ef5f1b2a077a53a48f36610223a7a0b82ad4de9

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui