I used to have a similar problem. For the black background problem and the change in Unity theme, this post solve my problem:
I finally figured out what caused this problem. At some point in time,
somehow I'd done something wrong with my gtk-3 settings. Remove the
rm -r ~/.config/gtk-3.0
and then logging out and in again solved my problem. Now all the themes look fine again.
Then my icons were even larger than yours, but recover the default theme configuration. I know this sounds kind of weird, but I managed to change the size of the icons by open a folder (any folder) with files inside (any type of files) and keep pressing Ctrl + Mouse Scroll, this change the size for all files inside and outside the folder.
Sometimes best solutions are the easiest.