Various style updates

[sanderson]

• Adjust top padding on code blocks
• Add bold color for body text
• Adjust search input field styling
• Adjust light theme left nav colors
• .article-content p code { whitespace: nowrap; font-style: normal; }
• <code>'s hover state when inside of a link is disconnected from the link.
• <hr>
• .article-content pre { margin: 2rem 0 2.25rem; }
• h2,h3,h4,h5,h6 { & + .highlight pre { margin-top: .5rem; }}
• .article-content img { margin-bottom: 2rem; }
• .article-content li p:last-child { margin-bottom: 0; }