Whe does the .css file number change every time you update it?

If you just had a standard .css filename you could link to that with custom pages rather than having a seperate .css file.

Is there some way to link to the UBBt .css file externally, regardless of filename?

To prevent browser caching. If you have just the standard filename, then if you make changes to it, some browsers will have the old css file cached and won't see the new changes. The number changes so when a change is made everyone is sure to download the new css file.

If you have an external style that you use in the pages, you could add that into the Header Insert area in the General Display options of the control panel.

Ah, so I can bypass the CP styles section altogether?

That would solve my earlier problem.