refactor: update scss variables

Author: mrholek

scss/_alert.scss

@@ -48,9 +48,9 @@
   $border: var(--#{$variable-prefix}alert-#{$state}-border-color, map-get($map, "border"));
   $color: var(--#{$variable-prefix}alert-#{$state}-color, map-get($map, "color"));
   $link-color: var(--#{$variable-prefix}alert-#{$state}-link-color, map-get($map, "link-color"));
-  // @if (contrast-ratio($background, $color) < $min-contrast-ratio) {
-  //   $color: mix($value, color-contrast($background), abs($alert-color-scale));
-  // }
+  @if (contrast-ratio(map-get($map, "background"), map-get($map, "color")) < $min-contrast-ratio) {
+    $color: var(--#{$variable-prefix}alert-#{$state}-color, mix(map-get($map, "color"), color-contrast(map-get($map, "background")), abs($alert-color-scale)));
+  }
   .alert-#{$state} {
     @include alert-variant($background, $border, $color, $link-color);
   }

scss/_buttons.scss

@@ -14,7 +14,7 @@
   vertical-align: middle;
   cursor: if($enable-button-pointers, pointer, null);
   user-select: none;
-  background-color: transparent;
+  background-color: var(--#{$variable-prefix}btn-bg, transparent);
   border: $btn-border-width solid transparent;
   @include button-size($btn-padding-y, $btn-padding-x, $btn-font-size, $btn-border-radius);
   @include transition($btn-transition);

scss/forms/_form-range.scss

@@ -28,7 +28,7 @@
     width: $form-range-thumb-width;
     height: $form-range-thumb-height;
     margin-top: ($form-range-track-height - $form-range-thumb-height) * .5; // Webkit specific
-    @include gradient-bg($form-range-thumb-bg);
+    @include gradient-bg(var(--#{$variable-prefix}form-range-thumb-bg, $form-range-thumb-bg));
     border: $form-range-thumb-border;
     @include border-radius($form-range-thumb-border-radius);
     @include box-shadow($form-range-thumb-box-shadow);