If value is not set, then AppStyling image is used automatically. For horizontal slider it is igsli_PlusFocusH.gif or igsli_MinusFocusH.gif, and for vertical slider it is igsli_PlusFocusV.gif or igsli_MinusFocusV.gif.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2