Change the owner of an offline database without bringing it online
This can't be done without bringing the database online, and this is not just a GUI problem. 

ALTER AUTHORIZATION ON DATABASE::test TO sa;


Yields:


  Msg 942, Level 14, State 4, Line 1
  Database 'test' cannot be opened because it is offline.


So, as one of the comments suggested, bring it online, set it to single_user if you need to, change the owner, then take it back offline.

ALTER DATABASE test SET ONLINE;
ALTER DATABASE test SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
ALTER AUTHORIZATION ON DATABASE::test TO sa;
ALTER DATABASE test SET MULTI_USER WITH ROLLBACK IMMEDIATE;
ALTER DATABASE test SET OFFLINE WITH ROLLBACK IMMEDIATE;


Though I have to wonder, why do you care who the owner is until you've brought the database online?