As above When you do your update with current style sheets
in place, the upgrade script does a very good job

Ive one little qwerk in the spoiler above.
But the examples above are more for after the event.
Say you are now running 7.3 but have 7.2 style sheets
that you want to import manually my first post, is a
little list of what to look out for.
The list looks worse than it actually is

Its very easy to update.
As you can use any upgraded style sheets as a guide

Hope that helps.