Springload changes for #240, #241 & #243

[katie-day]

Changes made:

• Change background colour from grey to white
• Add 24px bottom padding to breadcrumb and remove heading style
• Increase all headings weight to 600
• Reduce body weight to 300
• Remove Uppercase styling on sidebar heading
• Add border and change bg colour to grey in sidebar
• Divider line (between header and body) constrained to margins (rather than spanning whole page width).

Springload figma link: https://www.figma.com/file/QdmAlSacAoO23ru613kMhZ/IETF-UI-tweaks?node-id=78%3A19&t=lAmRHoslsYHVp0bU-0

From this:

to this: