Quick Edit not working with Custom Theme in SharePoint 2013

Type: List

View: Datasheet or AllItems > select quick edit

Problem: Datasheet view is showing grid lines, but unable to Edit the table when custom theme is applied. Works fine with out of the box SharePoint 2013 On Premise Theme.


Problem occurs when using custom theme.

Please note:

 - Active X is enabled/compatible on the master page.
 - Master page, default browser is IE8.
 - Quick Edit IS enabled in the list
 - Browser is IE8 (company supported, upgrade coming soon)

What do I need to add to the MasterPage, to ensure that datasheet views / quick edit works.