[ADD] dark theme for article preview

main/migrations/0030_mainarticlepage_is_black.py

main/models.py

...terns/molecules/how-we-work/work-article-preview.mustache

...ource/_patterns/organisms/main-articles-timeline.mustache

main/styleguide/source/css/atoms/heading.pcss

main/styleguide/source/css/molecules/articles.pcss

main/styleguide/source/css/style.pcss

main/templates/main/includes/work_article_preview.html