fix: wrong select option background on Firefox when dark mode is on

Author: mrholek

scss/_forms.scss

@@ -89,6 +89,15 @@ select.form-control {
   }
 }
 
+@-moz-document url-prefix() {
+  select.form-control option {
+    // Add background-color for elements in Firefox
+    @include themes($form-theme-map, $create: parent) {
+      background-color: themes-get-value("select-option-bg");
+    }
+  }
+}
+
 // Make file inputs better match text inputs by forcing them to new lines.
 .form-control-file,
 .form-control-range {

scss/themes/dark/variables/components/_buttons-forms.scss

@@ -15,6 +15,7 @@ $input-plaintext-color__dt:           $body-color__dt !default;
 $input-group-addon-color__dt:         $body-color__dt !default;
 $input-group-addon-bg__dt:            rgba($white, .09) !default;
 $input-group-addon-border-color__dt:  rgba($white, .09) !default;
+$select-option-bg__dt:                $bg-elevation-8 !default;
 
 // stylelint-disable-next-line scss/dollar-variable-default
 $form-theme-map: map-merge(
@@ -33,6 +34,7 @@ $form-theme-map: map-merge(
       "input-group-addon-color":         $input-group-addon-color__dt,
       "input-group-addon-bg":            $input-group-addon-bg__dt,
       "input-group-addon-border-color":  $input-group-addon-border-color__dt,
+      "select-option-bg":                $select-option-bg__dt
     )
   )
 );

scss/variables/components/_buttons-forms.scss

@@ -140,6 +140,8 @@ $form-group-margin-bottom:  1rem !default;
 $input-group-addon-color:         $input-color !default;
 $input-group-addon-bg:            $gray-100 !default;
 $input-group-addon-border-color:  $input-border-color !default;
+
+$select-option-bg: inherit !default;
 // scss-docs-end forms
 
 $form-theme-map: () !default;
@@ -158,7 +160,8 @@ $form-theme-map: map-merge(
       "input-plaintext-color":           $input-plaintext-color,
       "input-group-addon-color":         $input-group-addon-color,
       "input-group-addon-bg":            $input-group-addon-bg,
-      "input-group-addon-border-color":  $input-group-addon-border-color
+      "input-group-addon-border-color":  $input-group-addon-border-color,
+      "select-option-bg":                $select-option-bg
     )
   ),
   $form-theme-map