As per this blog by Gab Davis, customisations in connection V5 is not the same as V4.5
Now that is expected with just about any software upgrade though a list of upgrade requirements/changes for customisations just as there is for administration would have not gone amiss,
The nearest you will get is this blog by Paul Godby (which is very useful)
It gives a lot of excellent details, I personally found that the major changes between connections CSS v4.5 and v5 were ones of extra granularity caused by catering to the dynamic widths on the content (do not forget that your widgets will no longer be fixed width) and spruced up side menu in V5, so you should find that adding extra elements to your existing CSS rules will solve 90% of the problem the upgrade brings
In additional to the details provided by Paul, its worth noting that the open social widget standard seems to have been quietly dropped and iwidgets are back in favour (the basic open social widget wrapper for an XPage no longer works)
So where you might have had a basic open social widget XML of
you will most likely go back to the iwidget version of
The good news is the SSO works just fine in the iwidget.