See attached.

Workaround:

radCheckBox1.ButtonElement.MouseEnter += CheckMarkPrimitive_MouseEnter;
radCheckBox1.ButtonElement.MouseLeave += CheckMarkPrimitive_MouseLeave;


private void CheckMarkPrimitive_MouseLeave(object sender, EventArgs e)
{

    radCheckBox1.ButtonElement.CheckMarkPrimitive.Fill.ResetValue(BorderPrimitive.BackColorProperty, ValueResetFlags.Local);
}

private void CheckMarkPrimitive_MouseEnter(object sender, EventArgs e)
{

    radCheckBox1.ButtonElement.CheckMarkPrimitive.Fill.BackColor = Color.Red;
}