I had the hardest time trying to skin the scrollbar (which is a subcomponent) for the TextArea component. Finding no definitive answer in any one place, I did however piece together enough information to finally accomplish just that.
**Here’s the steps I took to do it… **
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I opened the HaloTheme.fla 
 (Path…Flash root installation folder>en>FirstRun>ComponentFLA>HaloTheme.fla
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Save as myTheme.fla (or any other name you choose) 
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Open Library (Ctrl+L) 
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Inside the Flash UI Components 2 folder navigate to Theme>MMDefault 
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Open each folder for the component you wish to skin and break it down to the core assets and modify to your liking 
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Save fla after all modifications are made 
**To Use Your Modified Skin **
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Open a new Flash document (Save As and name it as you wish…here I’ll use appliedTheme.fla) 
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Open the myTheme.fla file (or your choice) 
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Drag the Flash UI Components 2 [COLOR=crimson]folder from the myTheme.fla library[/COLOR] [COLOR=darkblue]to the stage of appliedTheme.fla[/COLOR] 
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You now have the Flash UI Components 2 folder in the appliedTheme.fla…just use the components from the Components Panel as before 
I’m sure there are other ways, but that worked for me. I know it has to possible with AS too. (I was able to do the TextArea background with AS but not the scrollbar). I did the TextArea with…
[AS]  // Change background color of TextField component instance myText_txt.setStyle(“backgroundColor”, “0x5a6970”)
myText_txt.setStyle(“borderColor”, “0x6e818a”)     /Outside Top and Bottom/
myText_txt.setStyle(“highlightColor”, “0xabc9d6”)  /Inside Top/
myText_txt.setStyle(“shadowColor”, “0xabc9d6”)     /Inside Bottom/
myText_txt.setStyle(“borderCapColor”, “0x6e818a”)  /Outside Left and Right/
myText_txt.setStyle(“shadowCapColor”, “0xabc9d6”)  /Inside Left and Right/
[/AS]