I installed i3 in my laptop last night and through following those guides posted by Code Cast on YouTube, I managed to get it set up and looking really nice, minus one thing, Qt applications. I'm running Kubuntu 15.10 so most of my programs are KDE specific, and all look very ugly because the default Qt theme is loading. How can I change this theme, using something along the lines of lxappearance (that's what Code Cast used to edit GTK themes)? I tried googling it and I only got results for KDE 4, and I tried everything I read, but nothing worked. If all else fails I'll fall back to using GTK applications, but I'd rather not bloat my system up too much in case I want to go back to plain Kubuntu.
So Qt is a bit different to GTK as far as themes go. I would recommend having a look at qtcurve. This is a config for numix on Qt that may interest you or at least give you a good starting point for creating your own custom theme.
This guide shows you how to install qtcurve in Ubuntu/Kubuntu/debian variants.
Hope this helps!