

Qt can use GTK+ styles, but the implementation has changed significantly in the recent year or so. Issue number two: Dropbox is written using Qt, more precisely Qt5. If not, make sure XDG_CURRENT_DESKTOP is set to Unity before Dropbox runs the Ubuntu version of the package does this internally.

So if you want to avoid part of the issues, use it, not the package Dropbox provides. This issue has been addressed by an updated /usr/bin/dropbox wrapper as shipped in Ubuntu's nautilus-dropbox package. It sometimes fails, as it's not been updated since 2015. Issue number one: Dropbox attempts to fit to whatever environment it runs under, so it does some autodetection work.

Unfortunately, most of the answers here are pure voodoo and do not address the actual underlying issues, of which there are two (I'm writing this in part so that I don't forget it myself the next time I'm dealing with this).
