CSS: Extract fonts to CSS variables

[ssddanbrown]

For easier customization and codebase reuse. Current SASS vars.

Docs Updates

• Update customising visuals page.
• Update advisory to warn of change in how fonts are customized.

Example Usage

<style>
	body {
	  --font-body: 'Noto Serif', serif;
	  --font-heading: 'Roboto', sans-serif;
	  --font-code: 'Source Code Pro', monospace;
	}
</style>

[ssddanbrown]

Added in 610ad0d