Theme based links is all about getting links from pages with the same topic (theme) that fits your niche (website). For that you first have to find sites that fits your topic and contact the site owner to convince him that a link would be beneficial for both sides... Those are hard to find but the benefit is much better than normal back links. This normally work great with pages from different locations. example your site is located in canada and you get a link from a page from the same niche from usa.