SharePoint Online - Theme not applying to 'Visitors' group

We have created a new SharePoint theme and have uploaded it to 'site master pages' and ensured that it is published as a major version in http://ift.tt/1w52dgK

The theme is in as approved and has been applied to the parent site and all subsites inherit that theme also. Users that are in the parent site group 'Visitors' cannot see the theme. The theme doesn't apply to them as they browse the page. However if we add users to the 'Members' or 'Owners' permissions group then the theme will be displayed correctly and applied.

Is there something we are missing. What can we do to ensure that users in the Visitors group can see the theme?

We are using Office 365 Business Premium formerly Office 365 Midsize Plan with SharePoint Online Plan 1.

Any help would be appreciated.

Thanks.