Created attachment 150681 [details]
GDB log showing where the buggy X11 command is issued by quadrapassel

repro steps:

1. from a terminal; launch one of these commands:

  quadrapassel --sync

OR, if you prefer:

  gdb --eval-command='break gdk_x_error' --eval-command=r --args quadrapassel --sync

2. select Settings::Preferences and switch to the "Theme" TAB
3. press CLOSE without changing anything
4. the following SIGTRAP happens immediately

Gdk-ERROR **: The program 'quadrapassel' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
  (Details: serial 136 error_code 3 request_code 10 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
aborting...
Trace/breakpoint trap

See the attached GDB log for details.