serving default jekyll-bootstrap blog with latest (3.0.0) jekyll will not load assets for blogs i.e. core-samples/lessons/2011/12/29/jekyll-introduction
Looks like with new verison Jekyll/Liquid front matter do not inherit theme variable from layouts, thats why page.theme.name is empty.

as workaround its possible to define theme for every post, or define ASSET_PATH directly in config.

btw - why not to define theme variable in config file to use in JB/setup ?

UPD: as for jekyll 3.0 layout variables now accessible with layout.variable. So updating JB/setup's capture of ASSET_PATH would resolve issue but only for posts.

I'm not sure why, but it still wont work for any layout: page pages.

Here is my workaroud

# JB/setup
{% capture ASSET_PATH %}{{ BASE_PATH }}/assets/themes/{{ site.JB.THEME }}{% endcapture %}

# _config.yml
JB :
  THEME : bootstrap-3