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Hello alantony
Welcome to Xoops! Yes it's that simple. Just change theme.html to your liking. Remember that you will have to do a full page reload to see your changes, and, if your running cache, you will also have to refresh the cache (by clearing cache folder or setting the necessary cached blocks or modules to 0)
I'm not running the latest version of XOOPS (which you probably downloaded) so I'll wait for someone else to help with the precise location.