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/assets/styles/bmc/custom/_buttons.scss b/src/assets/styles/bmc/custom/_buttons.scss
index b9b8073..3f11130 100644
--- a/src/assets/styles/bmc/custom/_buttons.scss
+++ b/src/assets/styles/bmc/custom/_buttons.scss
@@ -21,23 +21,23 @@
 }
 
 .btn-link {
-  fill: $primary;
+  fill: theme-color("primary");
   text-decoration: none !important;
 
   &:focus {
     box-shadow: $btn-focus-box-shadow;
   }
   &:hover {
-    fill: darken($primary, 15%);
+    fill: darken(theme-color("primary"), 15%);
   }
 }
 
 .btn:disabled {
-  color: $gray-600;
+  color: gray("600");
   fill: currentColor;
 
   &:not(.btn-link) {
-    border-color: $gray-400;
-    background-color: $gray-400;
+    border-color: gray("400");
+    background-color: gray("400");
   }
 }
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