Update use of Sass variables for better theming

Use Bootstrap color functions for theme-colors and grays instead
of directly referencing Sass variable to allow more flexible
theming.

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: Id08b77ff6df3bdf99400dcdfe964853706f1070f
diff --git a/src/components/Global/Search.vue b/src/components/Global/Search.vue
index b4be463..c385740 100644
--- a/src/components/Global/Search.vue
+++ b/src/components/Global/Search.vue
@@ -59,6 +59,6 @@
   left: 10px;
   top: 12px;
   z-index: 4;
-  stroke: $gray-400;
+  stroke: gray('400');
 }
 </style>