Workaround: change all RadForms to inherit the following custom base form
public class RadFormBase : RadForm
{
    protected override void OnLoad(EventArgs e)
    {
        if (this.IsDesignMode)
        {
            this.BackColor = Color.Empty;
        }

        base.OnLoad(e);
    }

    public override Color BackColor
    {
        get
        {
            return this.ElementTree.RootElement.BackColor;
        }
        set
        {
            if (value == Color.Empty)
            {
                this.ElementTree.RootElement.ResetValue(VisualElement.BackColorProperty, ValueResetFlags.Local);
            }
            else
            {
                this.ElementTree.RootElement.BackColor = value;
            }
        }
    }
}