Phase 2 work on boostlook UI

[rbbeeston]

To be filled in after review

• inline code in a colist needs background removed (example page)
• remove letter spacing from h2
• add space under TOC header to match spacing under h2s
• fix table heading styles on dark mode (ex: https://www.stage.boost.cppalliance.org/libraries/algorithm/)
• fix spacing for paragraphs within list items (ex: Notes on https://www.stage.boost.cppalliance.org/libraries/charconv/)
• <pre> blocks need rounded corners like <code> (ex: https://www.stage.boost.cppalliance.org/libraries/charconv/)