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I'm not familiar with new style profiles, but if you're using classic custom, you need to have all the BBC code to make changes. A url to a theme won't work because the Gaia system won't know what you're trying to tell it, and it can't superimpose an image and line all of your information up to it without the rest of the coding saying what info boxes should be present and where they should be. For example, if you're using a theme from TekTek.org, you'll need to copy and paste the code provided on the theme page- not the url to the preview image.
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