fixes #17385: add var for default link decoration

scss/_reboot.scss

@@ -146,7 +146,7 @@ blockquote {
 
 a {
   color: $link-color;
-  text-decoration: none;
+  text-decoration: $link-decoration;
 
   @include hover-focus {
     color: $link-hover-color;

scss/_variables.scss

@@ -76,6 +76,7 @@ $body-color:                 $gray-dark !default;
 // Style anchor elements.
 
 $link-color:                 $brand-primary !default;
+$link-decoration:            none !default;
 $link-hover-color:           darken($link-color, 15%) !default;
 $link-hover-decoration:      underline !default;