Make header margin an overridable setting

This provides more flexibility for writing themes.

Specific changes:

 1. `css/theme/template/settings.scss` adds the `$headingMargin` variable, set to `0 0 20px 0`, which is the hardcoded default before this pull request.
 2. `css/theme/template/theme.scss` uses $headingMargin to set margin on headers.