How do I theme these two parts?

They are "Left hand & right hand lines" in SkinStudio -Additional parts

Works great for the one on the right- but doesn't seem to affect the one on the left. Also I'm trying to theme the background of all the folders on the left- any idea how I do that?

OK, this is pretty easy. I see you are the latest Windows 10. I am assuming you have the latest WB and Skinstudio installed. 

Open your skin in SkinStudio and go to Theme Part Details, Command Bars. There you will see in the ListView area the "vertical Line" image. That is the left hand line.

Next, go to Edit Explorer/IE Shell Parts, Edit Additional Parts. The left and right lines, add them.

Next, go to Change Skin Colours and use the dropdown to go the the Show me the skin defined system colours area. In the far right (scroll to get their), you will find the colour settings you need to change now for Windows 10. You can change the explorer, folder bar and explorer text colours there. I am working on a new skin here myself, but haven't been able to see if those work yet, still designing in Windows 7 first. Let me know if this works out for you. 

.....ok

This worked perfectly! Thanks!!!

Glad you got it sorted out, and thanks for letting me know it worked. 

You're the man, man!

I have another question: do you know where windows stores the default images for folders and media files?  

Also, how do I get rid of the white lines inbetween my folders like in this pic: http://imgur.com/a/LAnMP ?

Sorry, not sure on those images smodis123, not something I ever had a need to look into.

As far as the white line, I have no idea. I color pick it, but that color is nowhere in the skin, so it is something Windows itself does. Andy ( neone6 ), may have some idea on this one though.

One of these should fix it

Thanks Andy, none of mine in that area match a color pick of that line though. It may very well be a color in that area, but Windows itself does something to it for sure to create the color of that line. Bummer. 

-Neil Banfield...