This question already has an answer here:
Occasionally, the default gtk theme gets broken; I can see it in some apps (e.g. Firefox) which seem to revert to the old and ugly gnome theme. But more evidently than that, I'm not able to successfully apply any new custom theme. Again, what I get is ugly gnome icons (in Wingpanel for example) and broken UI (missing buttons, gigantic toolbars etc.). The issue seems to be user-related: it doesn't occur in a guest session or if I launch Files as administrator. As a possible cause, I have played around with elementary-tweaks in the past. Now my question is: how can I troubleshoot this issue? How can I find which config file in my home is responsible for this?