Search showed up a previous post about this issue specifically re. QT theme, but that was in 2013 and I'm using a GTK+ dark theme in GNOME. So apologies for the repost but I think it's worth rasiing the issue again. This is a common frustration for those of us who prefer to use a dark theme.
Basically, text typed into the search box is invisible. It looks like you're forcing the search box background to be white, but taking the desktop theme colour for the text, which is also white. The same is true of the text boxes on the preferences page.
Maybe the fix isn't as easy as it seems: either use the theme colours for both foreground and background, or not at all.
Apart from that the linux client is great and big plus for Spotify.