In iphone.css magento also uses data URI but without quotes, try to do the same
background: #fafafa url(data:image/svg+xml;base64, PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZlcnNpb249IjEuMSIgeD0iMHB4IiB5PSIwcHgiIHdpZHRoPSI2cHgiIGhlaWdodD0iM3B4IiB2aWV3Qm94PSIwIDAgNiAzIiBlbmFibGUtYmFja2dyb3VuZD0ibmV3IDAgMCA2IDMiIHhtbDpzcGFjZT0icHJlc2VydmUiPjxwb2x5Z29uIHBvaW50cz0iNS45OTIsMCAyLjk5MiwzIC0wLjAwOCwwICIvPjwvc3ZnPg==) no-repeat;
In your case instead of
/url\\(\\s*(?!data:)([^\\)\\s]+)\\s*\\)?/
you should use
/url\\(\\s*(?![\"\']?data:)([^)]+)\\)/