I think it would be better if the site was related to your site because if it's not, the value of the links won't give any help to improved your rankings.
For SERP, it has no effect. However for traffic it is good. Just another "road" that leads to your site.
IMO people overestimate the importance of 'theme' from a link. Look at the backlinks of the sites that rank above yours. Would you rather have a link from a newer site with an identical 'theme' or a authority powerhouse that links to your site in passing...
There are a lot of curious answers in this thread. Personally I think that hosting and programming would fall under the same main 'high level' heading and would provide some benefit - certainly more than, as you say, links from a cookery site. But try checking some terms in http://labs.google.com/sets which gives some indication of what items google considers as grouped together.
There are a ton of blogs out there that don't even HAVE a theme - they're just lists of links to curious or interesting material. The search engines love these links. Draw your own conclusions.
Every link helps - but some help more than others. If you can focus on just on topic links - do it. But every links helps even if it just boosts PR. Boosting PR helps your inner pages rank better.