The blue hyperlink text color (as well as the normal black text) is too dark against the dark-gray background color of the success message upon a completed transfer. What it looks like:

Consider using a much brighter background color.

Alternatively, have you considered using a standard GtkInfobar widget instead, or a standard desktop notification, or some other way to display this? Maybe a folder GtkButton to "Open containing folder" in recent received files, too?