不要な記述を削除して、最適化した書き方に。

src/client/styles/scss/_override-bootstrap-variables.scss

@@ -22,30 +22,8 @@ $gray-600: lighten($dark, 10%) !default;
 $gray-700: lighten($dark, 5%) !default;
 $gray-800: $dark !default;
 $gray-900: darken($dark, 5%) !default;
-$grays: () !default;
-$grays: map-merge(
-  (
-    "50": $gray-50,
-    "100": $gray-100,
-    "200": $gray-200,
-    "300": $gray-300,
-    "400": $gray-400,
-    "500": $gray-500,
-    "600": $gray-600,
-    "700": $gray-700,
-    "800": $gray-800,
-    "900": $gray-900
-  ),
-  $grays
-);
-
+$grays: ("50": $gray-50) !default;
 $red: #ff0a54 !default;
-$colors: map-merge(
-  $colors,
-  (
-    "red": $red
-  )
-);
 
 //== Typography
 //