Hi Oliver,
Thanks for using Syncfusion products.
In order to change the header bottom border color and Header mouse hover color for metro theme, you can use the SetMetroStyle method. Please refer to the below code example,
Code example
GridMetroColors color = new GridMetroColors();
//Bottom border color
color.HeaderBottomBorderColor = Color.Green;
//Mouse hover color
color.HeaderColor.HoverColor = Color.Yellow;
this.gridGroupingControl1.SetMetroStyle(color);
Screenshot
Please refer the below UG and KB link,
Regards,
Mohanraj G