New $btn-link-focus-shadow-rgb for color modes customisation (#39119)

Author: giovannipiller

scss/_buttons.scss

@@ -170,7 +170,7 @@
   --#{$prefix}btn-disabled-color: #{$btn-link-disabled-color};
   --#{$prefix}btn-disabled-border-color: transparent;
   --#{$prefix}btn-box-shadow: 0 0 0 #000; // Can't use `none` as keyword negates all values when used with multiple shadows
-  --#{$prefix}btn-focus-shadow-rgb: #{to-rgb(mix(color-contrast($link-color), $link-color, 15%))};
+  --#{$prefix}btn-focus-shadow-rgb: #{$btn-link-focus-shadow-rgb};
 
   text-decoration: $link-decoration;
   @if $enable-gradients {

scss/_variables.scss

@@ -843,6 +843,7 @@ $btn-active-box-shadow:       inset 0 3px 5px rgba($black, .125) !default;
 $btn-link-color:              var(--#{$prefix}link-color) !default;
 $btn-link-hover-color:        var(--#{$prefix}link-hover-color) !default;
 $btn-link-disabled-color:     $gray-600 !default;
+$btn-link-focus-shadow-rgb:   to-rgb(mix(color-contrast($link-color), $link-color, 15%)) !default;
 
 // Allows for customizing button radius independently from global border radius
 $btn-border-radius:           var(--#{$prefix}border-radius) !default;