Thank you, pepak, for non-stupid answers!
Edit: After update complete.
... just to say, this update went very well. It took me a while to get userStyle.css the way I wanted it but this was nothing to do with the update process.
The only thing I would mention, to anyone else planning to install the new hack, is that I was expecting (wrongly) that the file /epub/userStyle.css would exist on the 505's internal memory after the update - ready to be edited. This is not the case, you have to create it. So don't assume the hack has not worked just because you can't see anything.
All in all I like this a lot, pepak. Thank you for your hard work.