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/PageSection.vue b/src/components/Global/PageSection.vue
index 5811998..6f44a3a 100644
--- a/src/components/Global/PageSection.vue
+++ b/src/components/Global/PageSection.vue
@@ -31,7 +31,7 @@
     content: '';
     display: block;
     width: 100px;
-    border: 1px solid $gray-300;
+    border: 1px solid gray('300');
     margin-top: 10px;
   }
 }