Add dark mode

[mostafa]

• https://github.com/gatewayd-io/docs/blob/main/_includes/nav_footer_custom.html
• https://medium.com/@derekkedziora/how-to-add-a-dark-mode-toggle-to-a-jekyll-site-a76dac00128d
• https://randomwits.com/blog/dark-mode-jekyll