For good measure, I've added on colours.css
background-repeat: no-repeat;to each of the items below
/* Icon images. Saved it, cleared the cache, cleared the front-end cache, and the problem automagically disappeared on all browsers I've tested. The only explanation I've got is that somehow the browser renderer got confused at some point and did not assume
no-repeatas the usual default for
background-repeat, although it's really strange why that should happen across all browsers (i. e. it could have been a Gecko thing, or a WebKit thing, but it's strange to affect both... and just some of the times).
3.1.9 had no such issues because the CSS was done differently.