Fantastic work was done! 
Many thanks, Arjan! Everything works both in default light and dark theme.
Share my configuration:
css.txt (5.7 KB)
head.txt (1.2 KB)
CSS customizations:
// Inline code colors
// Green thumbs-up
// Green assignee icon
// Grey or blue headers
// Center images
// Word wrap in code block + full content
// Fix huge topic margins
// Gitlab theme component font color fix (darker)
// Original <kbd> over Gitlab Markdown flavors theme component
// Line numbering in code blocks by @Arjan
Components installed:
https://github.com/discourse/discourse-search-banner
https://github.com/keegangeorge/discourse-markdown-flavors.git
https://github.com/pacharanero/discourse-topic-width-desktop.git