Ok I might know what your talking about. Kind of like it eats up massive amounts of cpu and ram and when you try and move it around the screen or what not it leaves traces of itself. That's usually a sign of the app crashing. You might want to try another program to change your gtk themes other than gnome-theme-manager but with that app not working it kind of defeats the purpose of running dropline gnome. But if you want you can try gtk-chtheme
for changing your themes on the fly.
That still leaves the problem of your icons because I don't know if there are any third party apps for that. I never use gnome theme manager to install themes. I just extract the themes and icons where they belong. For icons you download do:
$ mv icons.tgz ~/.icons
$ cd ~/.icons;tar -zxvf icons.tgz
or vice versa
And the same for themes except they go in your hidden theme directory. ~/.themes. A simpler way is with the gnome-theme-manager but don't install it with it just click the tab that says go to theme folder and you can just drag and drop the icon and theme folders to their correct location like above.
A reinstall of dropline should have fixed your issue. If not I'm sure they have some kind of support on irc or a forum maybe.