site: apply principles from ISEAL branding guidelines

The ISEAL Branding Guidelines Version 3.0 (June 2021) say we should
use Arial font for online material and it specifies the RGB values
to use for headings, sub-headings, and body text. It also specifies
four secondary colors to be used in support of the primary colors.

site/layouts/_default/term.html

@@ -6,7 +6,7 @@
                 <span class="">{{ .Params.title | markdownify }}</span>
                 {{- with .Params.required -}}
                 <div class="badge-required">
-                  <span class="badge bg-primary align-middle">Required</span>
+                  <span class="badge align-middle">Required</span>
                 </div>
                 {{- end -}}
               </th>