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I don't know if there are any tutorials specifically for NewBB, but what you need to do is play around with the module's template files.
If you have a moderately good grip of html and CSS, you'll be fine.
If you're going to do this through the XOOPS backend, don't forget that you should work on a 'cloned' template set - create the 'clone' set in admin>system>templates (which will need to then be set in admin>system>Preferences>general settings> Also, while on that page, don't forget to set 'Check templates for modifications ?' to 'yes')
Happy xoopsing!
Never let a man who does not believe something can be done, talk to a man that is doing it.