feat(style): minor style fixes

BREAKING CHANGE: old dark style `.well` is now `.well.dark`, the new `.well` will generate a lighter color scheme

- Update `.more-link` appearance
- New style `.download-btn` for download link inside posts
- Better search form and subscription form button width
- Update font stack
- Fix site navigation for mobile devices
- Fix `<hr>` inside `.entry-content`
- Better Infinite Scroll trigger button style