If you by chance are using the trial version of Windowblinds, the scrollbars will not be skinned (that's intentional).
I don't know if SKS free can tell that & disables scrollbar skinning as a result (I have always had an OD subscription), but if it doesn't, there are 8 images that comprise the scrollbar skin - the endbuttons (usually scrollbararrows.bmp or arrows.bmp), the endbutton/arrow glyphs (if used, not usually needed), the 2 scrollbar tracks (vertical & horizontal, usually scrollbarshaftvert.bmp & scrollbarshafthorz.bmp), the 2 "thumbs" or sliders (vertical & horizontal, usually scrollbarthumbvert.bmp & scrollbarthumbhorz.bmp) and the 2 scrollbar glyphs (often used, the "3 lines in the middle" of the sliders, usually scrollthumbglyphvert.bmp & scrollthumbglyphhorz.bmp). You can also have a glyph in each track/shaft image, but I don't think I've ever seen them used so I didn't count them.
The actual names will vary from skin to skin so you'll need to select individual images from the SKS preview or Explorer to see what they happen to be named.