Update default theme to use GitHub Markdown CSS

Since some of the markdown styles can get a little whacky, it would be nice if there was a clean template style using the GitHub Markdown CSS from the project: https://github.com/sindresorhus/github-markdown-css.

Start by locating the theme and static HTML template entry points in aglio, then review how existing themes are selected and rendered. Compare the current output with github-markdown-css; the work is done when a clean GitHub Markdown CSS-based default theme is available and renders the expected API Blueprint HTML.

Written by the indexing model from the issue text.