XNView doesn't have dedicated theme support, but a few elements can be customized from the Options menu. I recommend you access Tools > Options. There will be sections from where to customize various colors.
Additionally, users frequently post modifications to the XnView interface on the following support forum: http://newsgroup.xnview.com/viewforum.php?f=61
You will need to browse to the mentioned forum to obtain CSS code to change the appearance of the application.