Improve Primer.css styling

Github's Primer.css has been added to gollum for styling (implemented in https://github.com/gollum/gollum/pull/1393). This issue is meant to track any styling issues that can be improved upon, but also as an open invitation to improve the styling more generally.

• Use Primer to style the breadcrumbs
• Use Primer to style horitzontal rule
• Use Primer to style H1-H6 headings
• Box overlays or Dialog component
• Dark Mode
• Use Primer's Spinner component
• Cleanup of unused SCSS
• Replace editor selectors with Primer SelectMenus