A day or so ago, a upgrade to XFCE from 4.8 to 4.10 came through. I noticed some problems with theming: the panel loaded grey before turning to my theme-selected colour, panel icons appeared from the wrong theme, before (usually) changing to the correct theme icon, and highlighted text in Firefox was the default colour that is used whenever a theme is broken (a dark greyish blue).
At first I thought there must be a problem with my theme, but other themes had the same issues. Then I thought it must be a bug in XFCE.
While looking around for a solution, I came across a post on the XFCE forum which, while it didn't relate to the issues I was experiencing, seemed worth a try. And indeed, it did solve my problems. Something in the sessions cache was breaking the GTK2 theme.
The solution was to rename the following folder:
~/.cache/sessionsXFCE 4.10 is not a radical change from 4.8, but there are some notable differences. I shall write about those in another post.