Looking at your site and looking at your screen shot I can explain how that can happen very easily and think I know how to prevent it in the future.
Ok, when you save a template it writes to the MySQL database and a flat file in your UBB's "styles" folder with a file name kind of like ubbthreads_stock_1218749545.css as the file name.
Now say you back up your sites folders and a day it two later make a change to your sites template, even a small one, and then save it and such and so on and totally mess up your template or something. Now you may think..... well, I will just upload my backup of that style file and wont have to figure out how to get it back the way is was. In this instance your totally wrong as every time you change and save the style the number part of the file changes as it is a date and time code and will not match the time stamp in the MySQL database. This would cause what your screen shot shows.
Other reasons that this can happen is your server had a slight hick up when saving the file in the styles folder and a mismatch of the database and file name occurs, I have seen both happen, this will cause the same exact thing to happen to your web site UBB template to appear broken.
How to prevent this from happening:
Make a backup of your templates or export them to a file using the programs export function. If you botch your style you can delete it and import the good one fixing your site template.
OR.... Rick can change how he does the thing and eliminate the use of time stamps on files like this as they are un-necessary to have file names done this way for this application making it easier to manage file names. The best way would be to have it only look to the flat file if the database is unaccessible then look at the flat file for having your site look the way it should just for the purpose of showing your viewers an error screen.
Now for how I know all about this is it happened to me as described in the first instance. It also happened to a former admin and moderators who spammed all my users to join their community in an attempt to close my community. They didn't know enough about how the program worked and crashed the template and publicly made a statement that I hacked their community or aka was messing with them.
