Use correct variables
Normalize design
Remove redundant property
Make no assumptions about where heading classes will be applied

Some headings did not have the correct font weight.
Update styling of `alt-h3` to only remove trailing margin
Re-use CSS
Add missing `alt-h3` classes
Add docs on contributing to CSS
Remove misleading comments re. pixel sizes

As per https://github.com/guardian/developers-site/commit/b4c171f6494bbbb80d48fa8aaa720064bcd0d005\#commitcomment-6944581
Use `unset` mixin
Round value

No need for it to be so specific
Normalize line heights
Merge pull request #18 from guardian/oja-corrections

Tidy up