I got WARNING with run rake generate.

[woody1983]

ubuntu@ip-172-31-15-182:~/rails_project/tmp$ git clone git://github.com/imathis/octopress.git octopress
Cloning into 'octopress'...
remote: Reusing existing pack: 10372, done.
remote: Counting objects: 25, done.
remote: Compressing objects: 100% (24/24), done.
remote: Total 10397 (delta 7), reused 4 (delta 1)
Receiving objects: 100% (10397/10397), 2.75 MiB | 863.00 KiB/s, done.
Resolving deltas: 100% (4972/4972), done.
Checking connectivity... done.
ubuntu@ip-172-31-15-182:~/rails_project/tmp$ cd octopress/
ubuntu@ip-172-31-15-182:~/rails_project/tmp/octopress$ git clone https://github.com/sevenadrian/foxslide .themes/foxslide
Cloning into '.themes/foxslide'...
remote: Reusing existing pack: 641, done.
remote: Total 641 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (641/641), 952.82 KiB | 331.00 KiB/s, done.
Resolving deltas: 100% (241/241), done.
Checking connectivity... done.
ubuntu@ip-172-31-15-182:~/rails_project/tmp/octopress$  rake install['foxslide']
## Copying foxslide theme into ./source and ./sass
mkdir -p source
cp -r .themes/foxslide/source/. source
mkdir -p sass
cp -r .themes/foxslide/sass/. sass
mkdir -p source/_posts
mkdir -p public
ubuntu@ip-172-31-15-182:~/rails_project/tmp/octopress$ rake generate
## Generating Site with Jekyll
directory source/stylesheets/
WARNING on line 74 of /home/ubuntu/rails_project/tmp/octopress/sass/base/_syntax.scss: "html .gist .gist-file .gist-meta" failed to @extend ".sans".
  The selector ".sans" was not found.
  This will be an error in future releases of Sass.
  Use "@extend .sans !optional" if the extend should be able to fail.

WARNING on line 96 of /home/ubuntu/rails_project/tmp/octopress/sass/base/_syntax.scss: "pre" failed to @extend ".mono".
  The selector ".mono" was not found.
  This will be an error in future releases of Sass.
  Use "@extend .mono !optional" if the extend should be able to fail.

WARNING on line 112 of /home/ubuntu/rails_project/tmp/octopress/sass/base/_syntax.scss: "p code" failed to @extend ".mono".
  The selector ".mono" was not found.
  This will be an error in future releases of Sass.
  Use "@extend .mono !optional" if the extend should be able to fail.

WARNING on line 112 of /home/ubuntu/rails_project/tmp/octopress/sass/base/_syntax.scss: "li code" failed to @extend ".mono".
  The selector ".mono" was not found.
  This will be an error in future releases of Sass.
  Use "@extend .mono !optional" if the extend should be able to fail.

   create source/stylesheets/screen.css
Configuration from /home/ubuntu/rails_project/tmp/octopress/_config.yml
Building site: source -> public
Successfully generated site: source -> public

[bearqq]

I know little about these files but I solved this by editing the file "octopress\sass\base_layout.scss" and replacing it to the version I got a year ago. Pasted below.
http://pastebin.com/UhXQqE42