Inject CSS via `html_css_files` rather than inline styles in the sidebar

[pradyunsg]

See https://pradyunsg.me/furo/customisation/injecting/#css-or-js for the guidance from the theme for how to inject CSS.

Originally posted by @pradyunsg in #12326 (comment)

[nicoddemus]

Thanks @pradyunsg!

Marking @samjirovec here in case he would like to tackle this.