The possible solution that I can suggest is to use the ViewCellFormatting event and clear the applied BackgroundShape to the summary cell:
private void radGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
    GridSummaryCellElement summaryCell = e.CellElement as GridSummaryCellElement;
    if (summaryCell != null)
    {
        summaryCell.BackgroundShape = null;
    } 
}

Here is a sample code snippet how to replicate the problem: public RadForm1()
{
    InitializeComponent();

    ThemeResolutionService.ApplicationThemeName = "Windows8";
}

private void RadForm1_Load(object sender, EventArgs e)
{ 
    this.ordersTableAdapter.Fill(this.nwindDataSet.Orders);
    this.radGridView1.DataSource = this.ordersBindingSource;
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    
    GridViewSummaryItem summaryItem = new GridViewSummaryItem();
    summaryItem.Name = "ShipName";
    summaryItem.Aggregate = GridAggregateFunction.Count;

    GridViewSummaryItem summaryItem2 = new GridViewSummaryItem();
    summaryItem2.Name = "OrderID";
    summaryItem2.Aggregate = GridAggregateFunction.Sum;

    GridViewSummaryItem summaryItem3 = new GridViewSummaryItem();
    summaryItem3.Name = "ShipCountry";
    summaryItem3.Aggregate = GridAggregateFunction.Count;

    GridViewSummaryRowItem summaryRowItem = new GridViewSummaryRowItem();
    summaryRowItem.Add(summaryItem); 
    summaryRowItem.Add(summaryItem2); 
    summaryRowItem.Add(summaryItem3); 
    this.radGridView1.SummaryRowsBottom.Add(summaryRowItem);

    this.radGridView1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;
}