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data is stored in the database.. not on the server.. and data size varies.. so i can't give a definite answer there..
and yes you can change themes..
the XOOPS FAQ and documentation links in the left column will more than likely answer all your questions or use the search feature to find things before asking a question.. if u still can't find an answer then post in the forum, The regulars here try to help as much as possible but we hate repeating the same answers to questions that have been asked many times, so searching 1st will save on replies such as "use the search feature or read the FAQ" :)
and welcome to XOOPS :)