cotes2020/jekyll-theme-chirpy ? reverse-engineered prompt

Build me a clean, modern Jekyll theme for a technical blog that feels fast and polished on both desktop and mobile.

I want a homepage, post pages, category and tag browsing, a table of contents, search, dark and light mode, and a few nice quality of life features like pinned posts, last updated dates, code highlighting, math support, and embedded diagrams or media. It should also have a simple way to handle comments, feeds, SEO basics, and PWA style offline support.

Make it look minimal but still feature rich, with responsive styling and good typography. Please include sensible defaults so I can start writing posts right away, and make the theme easy to customize for my own site. If you need current Jekyll or plugin details, look up the latest docs online and use what fits best.