Before turning a Jekyll project into a theme

I currently have 3 repositories on GitHub that are Jekyll projects:

* egor-tensin.github.io,
* sorting-algorithms,
* blog.

They currently share a great deal of code in _layouts/, _includes/ and
css/.
I didn't want to bother creating a gem-based theme, but, as I've
recently learned, GitHub Pages support "remote" Jekyll themes, which
allows to simply host a theme on GitHub.

So I'd like to remove all of this common code from all 3 of the existing
repositories, and put it into this repository.
I'm going to do that by simply cloning the most feature-rich repository
(blog), and removing everything that isn't shared between the
repositories.

The alternative approach would be to use filter-branch, but I've done
that in the past, and that's too hard and error-prone for my liking.