The theme used for the blog is Readium, beautyfully designed by Sven Read.

If you need to modify the .sass files to change the look of your blog, it's pretty easy.

# First go to the readium directory
$ cd /path/to/my/blog/content/themes/readium

# Then go to the css directory
$ cd assets/css

# You need to have sass and bourbon installed, if not...
$ gem install sass
$ gem install bourbon

# Then install bourbon's file in the current css directory
$ bourbon install

# Open main.sass and edit the first line:
# - replace @import 'bourbon'
# - with @import 'bourbon/bourbon'
$ vim main.sass

# Finally, we need to compile the new file
$ sass main.sass:main.css

Reload your page and you should be good to go!

Hope this helps! :)