This is just an in-place upgrade to produce a roughly unchanged page
design. Ideally I'm going to need to install Sass and use that, because
Bootstrap 5 relies a bit more heavily on using its Sass sources if you
want to customise things (which I do), but for now loading standard
Bootstrap from the CDN is fine.
I still prefer Stylus over both Sass and LESS, but the industry seem to
have decided on using Sass, which probably means I'll be better off
porting my customisations to Sass in the long run. Oh well.