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/_base.scss b/src/assets/styles/bmc/custom/_base.scss
index 39a4aaa..5a6a11b 100644
--- a/src/assets/styles/bmc/custom/_base.scss
+++ b/src/assets/styles/bmc/custom/_base.scss
@@ -1,7 +1,7 @@
 dt,
 legend,
 label {
-  color: $gray-800;
+  color: gray("800");
   font-size: 14px;
   font-weight: 400;
 }