Use class-based syntax highlighting instead of inline CSS

[Aran-Fey]

Syntax highlighting in code blocks is currently done with inline styles like this:

<span style="color: #204a87; font-weight: bold">def</span>

This makes it impossible to customize syntax highlighting colors, which is a problem for dark theme users - code is unreadable on a dark background and impossible to restyle.

A far better solution would be class-based CSS rules like

<span class="syntax-darkblue">def</span>