If I remember correctly, around 4 to 1 or 5 to 1 ratio of regular and descriptive text to text links. You don't want these sites to be your only source of links. Have each site exchange links with sites that are not in the group and are unique to each site. That is a pretty good sampling of opinions. I still think the value of any link, outweights any potential negative value of it's offtopic nature.
Themes are nice, but there is room for the weird link too.
You'd be hard pressed to find sites where there isn't some link that is unrelated. When you look at all the se's that are using link counting, the value of the link is still the most important. If you work in the keyword into that link text, that reinforces the kw on the destination site. Themes are nice, but there is room for the weird link too. Include one of your targetted keywords in the text of the weird link.