Hi Chris
Thank you for contacting Syncfusion support.
Microsoft GroupBox control Back Color will be transparent by default. So once it is added to Form, it takes Parent Form back color as its BackColor. In this case, MetroForm Back color will be white and so GroupBox is in white as well. And as Microsoft Form BackColor is Light Gray, GroupBox control will adopt its BackColor as Light Gray.
We have not done any specific customization technique in this case. Kindly check with below screenshot and sample for your reference.
Screenshot
Kindly check with above solution and let us know if it helps
Regards
Kannan