descriptive docs variable naming (#24521)

* use descriptive docs btn naming

* rename `btn-bd-yellow` to `btn-bd-download`

* sort state vars

* sort state vars
  * info, warning, danger

* rename btn-bd-purple

* rename `.btn-bd-purple` to `.btn-bd-primary`

* align docs color variables

* generate $bd-purple-light color

* with http://razorltd.github.io/sasscolourfunctioncalculator/

* allow overide of docs variables

_includes/docs-navbar.html

@@ -59,5 +59,5 @@
     </li>
   </ul>
 
-  <a class="btn btn-bd-yellow d-none d-lg-inline-block mb-3 mb-md-0 ml-md-3" href="{{ site.download.source }}">Download</a>
+  <a class="btn btn-bd-download d-none d-lg-inline-block mb-3 mb-md-0 ml-md-3" href="{{ site.download.source }}">Download</a>
 </header>

assets/scss/_buttons.scss

@@ -2,7 +2,7 @@
 //
 // Custom buttons for the docs.
 
-.btn-bd-purple {
+.btn-bd-primary {
   font-weight: 500;
   color: $bd-purple-bright;
   border-color: $bd-purple-bright;
@@ -15,15 +15,15 @@
   }
 }
 
-.btn-bd-yellow {
+.btn-bd-download {
   font-weight: 500;
-  color: $bd-yellow;
-  border-color: $bd-yellow;
+  color: $bd-download;
+  border-color: $bd-download;
 
   &:hover,
   &:active {
-    color: $bd-graphite;
-    background-color: $bd-yellow;
-    border-color: $bd-yellow;
+    color: $bd-dark;
+    background-color: $bd-download;
+    border-color: $bd-download;
   }
 }

assets/scss/_variables.scss

 // Local docs variables
-$bd-purple:          #563d7c;
-$bd-purple-bright:   lighten(saturate($bd-purple, 5%), 15%);
-$bd-purple-light:    #cdbfe3;
-$bd-graphite:        #2a2730;
-$bd-yellow:          #ffe484;
-$bd-danger:          #d9534f;
-$bd-warning:         #f0ad4e;
-$bd-info:            #5bc0de;
+$bd-purple:        #563d7c !default;
+$bd-purple-bright: lighten(saturate($bd-purple, 5%), 15%) !default;
+$bd-purple-light:  lighten(saturate($bd-purple, 5%), 45%) !default;
+$bd-dark:          #2a2730 !default;
+$bd-download:      #ffe484 !default;
+$bd-info:          #5bc0de !default;
+$bd-warning:       #f0ad4e !default;
+$bd-danger:        #d9534f !default;

docs/4.0/getting-started/download.md

@@ -15,7 +15,7 @@ Download ready-to-use compiled code for **Bootstrap v{{ site.current_version}}**
 
 This doesn't include documentation, source files, or any optional JavaScript dependencies (jQuery and Popper.js).
 
-<a href="{{ site.download.dist }}" class="btn btn-bd-purple" onclick="ga('send', 'event', 'Getting started', 'Download', 'Download Bootstrap');">Download</a>
+<a href="{{ site.download.dist }}" class="btn btn-bd-primary" onclick="ga('send', 'event', 'Getting started', 'Download', 'Download Bootstrap');">Download</a>
 
 ## Source files
 
@@ -26,7 +26,7 @@ Compile Bootstrap with your own asset pipeline by downloading our source Sass, J
 
 Should you require [build tools]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/build-tools/#tooling-setup), they are included for developing Bootstrap and its docs, but they're likely unsuitable for your own purposes.
 
-<a href="{{ site.download.source }}" class="btn btn-bd-purple" onclick="ga('send', 'event', 'Getting started', 'Download', 'Download source');">Download source</a>
+<a href="{{ site.download.source }}" class="btn btn-bd-primary" onclick="ga('send', 'event', 'Getting started', 'Download', 'Download source');">Download source</a>
 
 ## Bootstrap CDN
 

index.html

@@ -17,7 +17,7 @@ layout: home
           Bootstrap is an open source toolkit for developing with HTML, CSS, and JS. Quickly prototype your ideas or build your entire app with our Sass variables and mixins, responsive grid system, extensive prebuilt components, and powerful plugins built on jQuery.
         </p>
         <div class="d-flex flex-column flex-md-row lead mb-3">
-          <a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/introduction/" class="btn btn-lg btn-bd-purple mb-3 mb-md-0 mr-md-3" onclick="ga('send', 'event', 'Jumbotron actions', 'Get started', 'Get started');">Get started</a>
+          <a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/introduction/" class="btn btn-lg btn-bd-primary mb-3 mb-md-0 mr-md-3" onclick="ga('send', 'event', 'Jumbotron actions', 'Get started', 'Get started');">Get started</a>
           <a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/download/" class="btn btn-lg btn-outline-secondary" onclick="ga('send', 'event', 'Jumbotron actions', 'Download', 'Download {{ site.current_version }}');">Download</a>
         </div>
         <p class="text-muted mb-0">