Add styling rules for good code examples in shared stylesheet This change primarily affects the C++ style guide: while a number of other style guides also use <pre> directly for code examples, the other style guides all use JS to prettyprint the code blocks, and the prettyprinter overrides the styles specified in the shared stylesheet. The exception is the AngularJS style guide, but it is already inconsistently formatted and this change does not make it any worse. There are also several other style guides other than C++ where good/neutral are rendered identically; this commit does not attempt to address those either.