Improve highlight.js default colours

[EilidhIsla]

Highlight.js defaults aren't ideal

162 made the code component accessible by using nihr blue, but it's difficult to distinguish

164 highlights the lack of contrast

Suggest we redo all the colours:

• classes and code elements coral if contrast is sufficient
• class label nihr blue
• element label - nihr green?

[bartfeenstra]

Keep in mind we're currently highlighting HTML and Nunjucks (through Jinja2/Twig highlighting), with CSS and JS available but not practically used yet. That means there'll be a few combinations of syntaxes we should test against :)