Add CSS variable example

src/default.sass

@@ -1 +1,11 @@
 @import '~bootstrap/scss/bootstrap'
+
+// Set the CSS variable --primary to the value of the bootstrap variable $primary.
+// The value of a bootstrap variable is determined at build time. After building, this
+// file will be converted to a file default.css with the content:
+//   * {
+//     --primary: #0d6efd
+//   }
+
+*
+  --primary: #{$primary}

src/solar.sass

@@ -189,3 +189,14 @@ $btn-close-color:                       $body-color !default
 $pre-color:                         inherit !default
 
 @import '~bootstrap/scss/bootstrap'
+
+// Set the CSS variable --primary to the value of the bootstrap variable $primary.
+// The value of a bootstrap variable is determined at build time. After building, this
+// file will be converted to a file default.css with the content:
+//   * {
+//     --primary: #b58900
+//   }
+
+
+*
+  --primary: #{$primary}

src/styles.sass

@@ -1 +1,7 @@
 /* You can add global styles to this file, and also import other style files */
+
+// Set the background color to the value of the CSS variable --primary. 
+// The value of a CSS variable may change at runtime if a different stylesheet is loaded.
+
+body
+  background-color: var(--primary) !important

src/united.scss

@@ -56,3 +56,15 @@ $table-dark-bg:               $dark !default;
 $table-dark-border-color:     darken($dark, 5%) !default;
 
 @import '~bootstrap/scss/bootstrap';
+
+// Set the CSS variable --primary to the value of the bootstrap variable $primary.
+// The value of a bootstrap variable is determined at build time. After building, this
+// file will be converted to a file default.css with the content:
+//   * {
+//     --primary: #e95420
+//   }
+
+
+* {
+  --primary: #{$primary};
+}

src/vapor.scss

@@ -168,3 +168,14 @@ hr:not([size]) {
 }
 
 @import '~bootstrap/scss/bootstrap';
+
+// Set the CSS variable --primary to the value of the bootstrap variable $primary.
+// The value of a bootstrap variable is determined at build time. After building, this
+// file will be converted to a file default.css with the content:
+//   * {
+//     --primary: #6f42c1
+//   }
+
+* {
+  --primary: #{$primary};
+}