vBulletin Overwriting Styles
 

I've got an somewhat detailed question that I hope I can get to make sense.

I have a theme which I use as a Parent theme for quite a few sites, and I find updates that I'd like to make to it at times. I don't want to go to each site, edit each of the parent themes files again and again (pretty tedious work). Is there some way I could say download the .xml of this parent theme. then upload it to overwrite the parent themes on other sites.

A more 'real-world' example:
Site X is running Parent Theme 1.1 (new and updated)
Site Y is running Parent Theme 1.0
Site Z is runnign Parent Theme 1.0

Could I somehow download the Parent 1.1 theme from Site X, and then on Y & Z overwrite just the files that have changed from 1.0 to 1.1.