This is the section where you can change the scrollbars:
<SCROLLBAR>
<Entry Name="HorizontalLeftArrow" Bitmap="Scrollbar_HorizontalLeftArrow.png" NumberImages="3" Margins="2,2,2,2" />
<Entry Name="HorizontalRightArrow" Bitmap="Scrollbar_HorizontalRightArrow.png" NumberImages="3" Margins="2,2,2,2" />
<Entry Name="HorizontalHandle" Bitmap="Scrollbar_HorizontalHandle.png" NumberImages="4" Margins="10,0,10,0" />
<Entry Name="HorizontalBackground" Bitmap="Scrollbar_HorizontalBackground.png" NumberImages="4" Margins="10,0,10,0" />
<Entry Name="VerticalUpArrow" Bitmap="Scrollbar_VerticalUpArrow.png" NumberImages="1" Margins="2,2,2,2" />
<Entry Name="VerticalDownArrow" Bitmap="Scrollbar_VerticalDownArrow.png" NumberImages="1" Margins="2,2,2,2" />
<Entry Name="VerticalHandle" Bitmap="Scrollbar_VerticalHandle.png" NumberImages="1" Margins="0,0,0,0" />
<Entry Name="VerticalBackground" Bitmap="Scrollbar_VerticalBackground.png" NumberImages="1" Margins="0,0,0,0" />
<Entry Name="Stump" Bitmap="Scrollbar_Stump.png" Margins="1,1,1,1" />
</SCROLLBAR>
For each scrollbar you can add max from 1 to 4 images, and for margins is to adjust the size of scrollbar on the skin, like croping image to fit your screen, there you need to be carefull with the size of the image and how far you want to strech it.
Hope this will help you there, to fix.