Allow users to select Highlightjs theme

There is currently no way for a user to select a highlight js theme. The result is we have arguments over which theme should be configured for the site (using the theme picker component) - some like light, some dark etc.

Even just being able to set site wide light and dark themes would be an improvement.

Also, the version of highlightjs is out of date - which means we are missing out on a ton more themes.