Hi Darshana,
If you want to change groupbox header style to add background colour than please find the example SASS style below;
//Add styles for group box header
.mx-groupbox-header{
color: #fff;
border-color: blue;
background: blue;
.glyphicon{
font-size:12px;
}
}
//Add styles for group box body
.mx-groupbox-body{
border:0;
padding: 10px;
}
I will recommend you to go through Style your App with CSS learning path to get better understanding.
Hope this helps!