Change background color in dark mode - login page

When the default theme was in dark mode, the login page showed a very
light background color in mobile.

Changelog: changed

app/assets/stylesheets/page_bundles/login.scss

@@ -286,7 +286,11 @@
 }
 
 @include media-breakpoint-down(sm) {
-  .sm-bg-gray-10 {
+  .sm-bg-gray {
     @include gl-bg-gray-10;
+
+    .gl-dark & {
+      background-color: var(--gray-100);
+    }
   }
 }

app/views/layouts/devise.html.haml

@@ -13,7 +13,7 @@
             = render "layouts/flash"
             - if custom_text.present?
               .row
-                .col-md.order-12.sm-bg-gray-10
+                .col-md.order-12.sm-bg-gray
                   .col-sm-12
                     %h1.mb-3.gl-font-size-h2
                       = brand_title