RadSplitter is loaded with the gray background when configured with Default/Silks/Bootstrap skins in RenderMode="Classic".

The issue applies to RadFileExplorer and RadEditor file browser dialogs too which use RadSplitter inside.

The background is white in version 2017.1.118.40 and earlier.

Workaround:
Switch to RenderMode="lightweight"

or 

RadSplitter: Apply the BackColor property to RadSplitter or its RadPane items.
RadFileExplorer: 
ASPX:
<telerik:RadFileExplorer ID="RadFileExplorer1" runat="server" RenderMode="Classic">
    <Configuration ViewPaths="~/"  />
</telerik:RadFileExplorer>
Codebehind:
using Telerik.Web.UI;

public partial class Default2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        (RadFileExplorer1.Splitter.GetPanes()[0] as RadPane).BackColor = System.Drawing.Color.White;
        (RadFileExplorer1.Splitter.GetPanes()[1] as RadPane).BackColor = System.Drawing.Color.White;
    }
}