Bootstrap iso got removed in upstream theme

[capital-G]

sylhare/Type-on-Strap#549 broke our styling, see

The vendored bootstrap is now replaced with a CDN which now results that the css of our theme is not loading as expected

GET
https://supercollider.github.io/assets/css/vendor/bootstrap-iso.min.css
[HTTP/2 404  22ms]

I naively tried to re-insert the css theme via the cdn

diff --git a/_includes/default/head.html b/_includes/default/head.html
index 4d01d24..d51cab5 100644
--- a/_includes/default/head.html
+++ b/_includes/default/head.html
@@ -44,8 +44,8 @@
     <link rel="alternate" type="application/rss+xml" title="{{ site.title | default: 'SuperCollider latests posts' }}" href="{{ 'feed.xml' | absolute_url }}"/>
 
     {% if site.bootstrap %}
-    <!-- Bootstrap-4.1.3 isolation CSS -->
-    <link rel="stylesheet" type="text/css" href="{{ '/assets/css/vendor/bootstrap-iso.min.css' | relative_url }}">
+    <!-- Bootstrap 4.1.3 CSS. -->
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.1.3/dist/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
 
     <!-- see https://getbootstrap.com/docs/4.6/getting-started/introduction/#separate -->
     <script src="https://cdn.jsdelivr.net/npm/jquery@3.5.1/dist/jquery.slim.min.js" integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj" crossorigin="anonymous"></script>

but this breaks the dark theme

I'll add a quickfix via a version restraint of the theme to 2.5.0 so we do not update to the newest version of the theme immediately with each deployment.

It would still be great if we could get in sync w/ upstream again ;)

Maybe @sylhare can provide some advice for a migration path? I don't have any clue about jekyll.