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go to admin>pref>general settings and you will see on something called default thing...(8th option). Change it from default to some other one and see that the look of the site changes. If you dont like any of the ones listed then you can download a theme from this website and then upload it into the themes directory and select that with the procedure above and your site has the new look like the theme you selected.
Hope that helps
cheers
prasad..