Thank you very much for your help Davebax and LightStar!
DarkSide Two was the only 'dark' skin I could find that would display the min/max/close buttons in FireFox, they mostly disappeared on EVERY other dark skin.
I created the 2px wide by 26px long bmp image and uploaded it as below:
http://s9.postimg.org/f6lx58f1b/menu2.png

That didn't work, so I tried the right hand line, that was still white. Finally, I used the same file for 'top area on libraries', and that DID work. (saving/applying/restarting explorer and usually rebooting at each step). So now it looks like:
http://s11.postimg.org/h53z8ykfn/phase2.png

Areas in red didn't work, but blue area did change. So I know the 2x26 bmp file works.
If you have any ideas I'd really appreciate it. Failing that, I'll assume it's a problem with DarkSide Two. I tried the skin "Ion" and one of the firefox buttons appears, and I bet I can apply what I've learned in SKS to make the other two appear. Or just use right click menu to minimize it. Either way, I think I've learned enough about WB and SKS to tweak a master skin to my liking. I assume 'master skins' are so called because they are done correctly. 
Thanks again, haven't quite got it right but I am happy to learn the programs, for $10 they are really powerful and well written.