scook
To reduce the number of possibilities, I load Tungsten into the theme editor and look at only the modified entries. The filters are on the theme editor view tab.
 
Sometimes you can get lucky by searching for the name. This works in the case of the "Add Track" buttons especially when looking at only modified entries. There are two "Add Track button" images in the Track View > Add Track Flyout.
 
The track control list drop is a little harder to find. I looked at modified images with more than 2 states in the Track view. I knew it was an image with more than 2 states because the menu changes color when the mouse hovers over it. I found it in Track View > Track Pane and it goes by the unfortunate name of "Edit Filter."

Thanks Scook. That's got one of them, and given me a a better idea of finding the other.
Unfortunately since returning to Mercury, when I look at my Red scheme I can see I lost some of the transparent layers, meaning I gained lots of white borders, so I need to go back through everything again. 
 

Make sure to use an image editor which supports transparent backgrounds like Paint.net (not MS Paint) or GIMP

Yeah I did, I was adding a layer, then colour burning to keep the texture. It was when I was merging the layers it seems I lost the transparent layer. Dumb mistake, I use transparencies for work a lot. I use Sketchbook Pro

zoffmeister
Modifying Mercury removing strip colours must be an oversight. At the moment Theme Editor is very much a Beta product. So much is still wrong. I hope they are working hard on this.

I'm not sure. The tracks after the end tab have track colour show under a translucent gray. In the theme editor, maybe there was an issue having it have a transparent layer to edit. With Tungsten this would have been really dark, so may not have been very functional.
 
Editing yesterday made me realise why you couldn't simply have a a fader to turn up or down the darkness of the of the background, as the buttons are in a different form. I think they could have implinted say 6 different values of light and darkness, so you could step through them. Maybe with two or three options of highlighted button colours along with it.