Change style of section separators in the demo pages

[waldyrious]

• Change style of section separators in the demo pages: make them stand out as outside the page content flow; use colors that provide adequate contrast (WCAG-compatible)
• Adjust styling of demo page dividers to work in the reset demo
  • The default font-weight and margin-bottom properties for headings are reset from the typical browser defaults, so they need to be specified explicitly
  • Images get floated, so the section dividers need a clear property