You will need to put the following images (you can get them in /usr/share/icons/...)
inside of the following folders
.icons is a hidden folder so you will need to check to view hidden files.
You will also notice that the .icons folder isn't there...but that is because you have to create it.
So you will need to create all of those folders
Quit Dropbox and then restart it and the new icons will be used...of course if you use a different theme then you need to make a folder for that theme.