script: Fix background color of Acid2.

There were two problems here: (1) we did not process style sheets with an
unexpected `rel` attribute but a correct MIME type; (2) we did not
consider `none` a valid value for the `background` property.
auto merge of #1905 : pcwalton/servo/acid2-fixes, r=SimonSapin

There were two problems here: (1) we did not process style sheets with an
unexpected `rel` attribute but a correct MIME type; (2) we did not
consider `none` a valid value for the `background` property.

r? @SimonSapin