css/style.css: specify default font size/line height.
Switch to classes.

Also remove the unused "wrapper" class.
SyntaxHighlighter: fix a few CSS issues.

* add missing unprefixed `border-radius` property
* `background-color: none` isn't valid so use `transparent` in its place
style.css: change the order of selectors; no functional changes.
Move common property to a new selector.
Use the shorthand background property.