I sometimes connect my laptop to an external monitor with an HDMI cable, and when I later unplug the cable, it causes a problem with the graphical theme in Xubuntu (14.04).

The window panes, panel, buttons etc. look ugly, and I am unable to change the theme by going to Settings Manager -> Appearance -> Style (I mean the style is on Greybird, as it should be, but it doesn't change when I click on different styles). This only seems to happen sometimes after I unplug the HDMI cable (not every time).

Screenshot of ugly appearance:

Screenshot of normal appearance:

The only way I can fix it is to log out and back in again, after which the appearance is back to normal. I assume something is crashing (gtk?). Can anyone explain what might cause this, or how I could track down the cause?

Edit: I use xmonad as a window manager, and restarting xmonad doesn't seem to help at all.