Update Timer with Bootstrap modal, styling
Add comments to describe selector blocks
Introduce default Bootstrap vars
Include improved Octicon styling for slide uses
Update slide heading, main text appearance

Reduce font weight for less heavy look, provide more contrast against
bolded text and Octicons
Cleanup excess white space
Simplify GH-API injected teacher name
Cleanup teacher slide styles
Merge pull request #288 from github/styling-refresh

Update style, tidy custom components