Hi Maryam,
You could create a custom class to achieve this. It is however important to follow DG2's structure. So, in order to change the styling of the headers, you could create something like this:
.customdatagridheader {
.table .th .column-header span {
color: red;
}
}
The above example will change the color of your header text to red. You can obviously change this to your liking, and adjust all the other text properties too.
Make sure to add this class (in the example above this would be 'customdatagridheader') within your DataGrid 2 Appearance tab and voila, you're all set.