I recently upgraded to Ubuntu 15.10, and now themes aren't being applied correctly to the UI. Fonts are rendered inconsistently, with elements appearing in what seems an old GTK default. My customized themes/icons/fonts are not loaded, so parts of the interface are rendered with the default Ubuntu theme (Ambiance) and other parts, with an ugly old-looking theme. The scaling factor configured for UI fonts is not applied.
I already tried creating new, empty user accounts, but the problem with the font rendering is still there. I even made a fresh installation of Ubuntu 15.10, but that didn't solve the issue, either.
I have noticed that sometimes the problem is solved by forcing Compiz to restart (and Nautilus for the desktop icons), executing these commands:
killall compiz killall nautilus nautilus -n &
I didn't find it easy to describe the problem, so maybe these screenshots that could clarify the explanation: