[TYPO3] create/manipulate CSS-file

[Georg Ringer]

Hello,

How can I do the following thing:
I have got some records with one image. After saving the record I want to 
create/edit a CSS file to add one line including the uid of the record and 
the path to the image like
.my-extension-ID  { background:transparent 
url(uploads/my-ext/path-to-file) }

I know about hooks and have already built some but how to do this with the 
CSS file?

many thanks for links/hints

georg