Reset select:disabled opacity and null-ify color to fix rendering dif…

…ferences in Chrome

scss/_reboot.scss

@@ -420,24 +420,26 @@ textarea {
 }
 
 // Remove the inheritance of text transform in Firefox
-
 button,
 select {
   text-transform: none;
 }
-
 // Set the cursor for non-`<button>` buttons
 //
 // Details at https://github.com/twbs/bootstrap/pull/30562
 [role="button"] {
   cursor: pointer;
 }
 
-// Remove the inheritance of word-wrap in Safari.
-// See https://github.com/twbs/bootstrap/issues/24990
-
 select {
+  // Remove the inheritance of word-wrap in Safari.
+  // See https://github.com/twbs/bootstrap/issues/24990
   word-wrap: normal;
+
+  // Undo the opacity change from Chrome
+  &:disabled {
+    opacity: 1;
+  }
 }
 
 // Remove the dropdown arrow in Chrome from inputs built with datalists.

scss/_variables.scss

@@ -796,8 +796,8 @@ $form-select-indicator-padding:     $form-select-padding-x * 3 !default; // Extr
 $form-select-font-weight:           $input-font-weight !default;
 $form-select-line-height:           $input-line-height !default;
 $form-select-color:                 $input-color !default;
-$form-select-disabled-color:        $gray-600 !default;
 $form-select-bg:                    $input-bg !default;
+$form-select-disabled-color:        null !default;
 $form-select-disabled-bg:           $gray-200 !default;
 $form-select-disabled-border-color: $input-disabled-border-color !default;
 $form-select-bg-position:           right $form-select-padding-x center !default;