When you use Internet Explorer 8 to view a SharePoint Online site that has a theme applied in Office 365, the default theme is displayed instead of the configured theme.
This issue occurs because recent changes were made to render themes in the browser instead of on the server. Office 365 is designed to work with the current or immediately preceding version of Internet Explorer. Therefore, Internet Explorer 8 can't render the themes.
Article ID: 3061085 - Last Review: Dec 21, 2016 - Revision: 1
Microsoft SharePoint Online