Avoid too much copy and paste in stylesheet

src/theme.scss

@@ -2169,8 +2169,11 @@ tag-input {
     }
 
     .has-background {
+
+        $darkBackgroundColor: #000000e0;
+
         .card {
-            background-color: #000000e3;
+            background-color: $darkBackgroundColor;
             border-radius: 2px;
         }
 
@@ -2183,11 +2186,11 @@ tag-input {
         }
 
         footer {
-            background-color: #000000e3;
+            background-color: $darkBackgroundColor;
         }
 
         .btn-default {
-            background-color: #000000cf;
+            background-color: $darkBackgroundColor;
             border-color: #868686cf;
             color: white;