Accommodate new system.css status message background-image

Somewhere recently core made the change to add icons via div background-images. This is fine if the core icons don't clash with the styling of themes, but it does clash for Corolla. The greens don't match and the positioning of status messages don't accommodate it. Either the colors should be tweaked and some padding / positioning added to accommodate the icon or the icons should just be themed away (my preferred solution). Happy to supply a patch if necessary. I love this theme. : )