Does that mean for the sake of SEO? Most of the time themes are mentioned at the footer of wp blogs.

You'll have to be specific If you expect any sensible help.

• Keyword theme?
• Overall niche theme?
• Blog template theme?

Help us, help you.

Combination of anchor texts on backlinks and onsite content factors.

My opinion based on results - does the algorithm really look at a page and say "this website is in the x niche" and classify it as such? Not really. It's still all content and links.

I've re-purposed branded sites and ranked extremely well in niches that's completely unrelated to what the site was originally for. This tells me that google still has no idea of context.

I would add two co-citation factors to what KyGunator said:

- The surrounding content of the incoming links. A site can rank for keywords surrounding the anchor text of the incoming links it's receiving, instead of the actual anchor.

- It's been said that search engines tend to find similarities between site A and site B, when they are often linked by site C, even if sites A and B are not linking each other. Like: if A and B are often linked from the same place, they must be related in some way.