Unable to Modify BackColor of Button and Header

By default for ButtonCell and HeaderCell, the background specified in the Visual Style is given preference; however, you can disable this setting and enable the background color by completing the following steps.

Set ButtonCell.FlatStyle property to Standard.
Set ButtonCell.UseVisualStyleBackColor property to False.
Set ButtonCell.Style.BackColor property to the desired color.

You can set the same properties for ColumnHeaderCell and HeaderCell also. You can do the same for using BackColor Gradation and BackColor Pattern.

For more information, see the following topics:

Change BackColor (HeaderCell)
Change BackColor (ButtonCell)
BackColor and ForeColor