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In my experience, the difficulty varies much depending on your HTML (and Smarty) knowledge and the complexity of the template.
TemplateMonster has a habit of using MANY tables and images that look cool, but are difficult to get your head around.
You should consider very carefully if the content that YOU have, suits the template. If it doesn't, the theme will look bad. On
http://xoops.web-udvikling.dk I have a couple of themes made from TemplateMonster templates. They are pretty ok, but I've seen a couple of templates that are just not suitable for a XOOPS theme due to the dynamic content.
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