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/AppHeader/AppHeader.vue b/src/components/AppHeader/AppHeader.vue
index a69d6d9..7f18e1d 100644
--- a/src/components/AppHeader/AppHeader.vue
+++ b/src/components/AppHeader/AppHeader.vue
@@ -179,7 +179,7 @@
   }
 
   .nav-item {
-    fill: $light;
+    fill: theme-color('light');
   }
 
   .navbar {
@@ -194,7 +194,7 @@
 
     .helper-menu {
       @include media-breakpoint-down(sm) {
-        background-color: $gray-800;
+        background-color: gray('800');
         width: 100%;
         justify-content: flex-end;
 
@@ -217,7 +217,7 @@
   }
 
   .nav-trigger {
-    fill: $light;
+    fill: theme-color('light');
     width: $header-height;
     height: $header-height;
     transition: none;
@@ -227,12 +227,12 @@
     }
 
     &:hover {
-      fill: $light;
-      background-color: $dark;
+      fill: theme-color('light');
+      background-color: theme-color('dark');
     }
 
     &.open {
-      background-color: $gray-800;
+      background-color: gray('800');
     }
 
     @include media-breakpoint-up($responsive-layout-bp) {