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i might be wrong here, but i think you could remove the copyright without any comebacks (ok some might say a bit disrespectful to the designer) but in reality i think having various different copyright notices visible all over your site looks un professional and messy, especially if you have copyright on the theme, and then some modules display a different copyright etc..
as long as you leave credits in the source i can't see a problem (it's released under GPL anyway i think)
and i think that the theme itself can't be copyrighted due it containing XOOPS code which makes you free to do what u want with it as long as credits are left intact in the source.. the only thing that the copyright notice affects in themes is the images themselves. again i maybe completely wrong here. but no doubt someone with more knowledge will correct me