taggit / autocomplete - add backgrounds to overlay items
taggit / autocomplete - add backgrounds to overlay items

- original backgrounds removed in client/scss/vendor/jquery-ui-1.10.3.verdant.css 4eb35dbc0a6588d3a7e0ae083f14f7c8faf4adde via #8419
- also ensure the dropdown items show correctly in high contrast mode
- fixes #9128

rsalmaso/wagtail,rsalmaso/wagtail,wagtail/wagtail,thenewguy/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,thenewguy/wagtail,thenewguy/wagtail,zerolab/wagtail,zerolab/wagtail,zerolab/wagtail,zerolab/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,wagtail/wagtail,zerolab/wagtail,wagtail/wagtail