That's up to you.
Armed with the minified BootCSS -- often a tiny fraction of the size of
your site's total CSS load, especially if you're using a WordPress or
Bootstrap theme -- it's trivial to just load the styles you need, defer the
rest of the CSS until after the page renders.
In some cases -- a simple site like
WhatCSS.info
-- the BootCSS is all the site needs to be fully functional. With only 5% of the
original CSS from the standard Bootstrap 4 distribution needed to use the site,
we just put the CSS inline.
Good enough for a technology demo, maybe doesn't quite meet your actual needs.
There's an example of how to do this in the next section, but an easy
variation of this might be to inline the BootCSS and defer the
load of the rest of the CSS. This way, the page becomes immediately
interactive, but the styles needed for additional exploration
come along shortly thereafter.
This approach also alleviates the need to make any hard choices when it comes
to cutting CSS, especially if you're just loading some random theme off a CDN.
With WhatCSS having an API/CLI and being opensource and whatnot,
it can be integrated to a build process,
automating the BootCSS generation.
More sophisticated users might want to divide their CSS into two parts,
the first, the BootCSS, essential for displaying the site, and then the
rest of the stuff. With both files on a CDN, the second can be safely
deferred... or even sub divided into tiny, chunked deferred pieces.