Add more themes

[neroist]

Add the following themes:

• CLMaterial: https://github.com/24aitor/CLMaterial
• CometCSS: https://marcbruederlin.github.io/comet/
• Concrete.css: https://concrete.style/
• Flat UI: https://designmodo.github.io/Flat-UI/
• FurtiveCSS: https://www.furtive.co/
• GutenburgCSS: https://github.com/BafS/Gutenberg#readme
• HypCSS: https://github.com/dohliam/dropin-minimal-css/blob/gh-pages/min/hyp.min.css (repo was deleted)
• PaperCSS: https://www.getpapercss.com/
• Ok CSS: https://okcss.netlify.app/
• Terminal CSS: https://terminalcss.xyz/
• classless.css: https://classless.de/
• spcss: https://github.com/susam/spcss
• hack.css: https://github.com/egoist/hack