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/_alert.scss b/src/assets/styles/bmc/custom/_alert.scss
index 0b7b518..1d28f2a 100644
--- a/src/assets/styles/bmc/custom/_alert.scss
+++ b/src/assets/styles/bmc/custom/_alert.scss
@@ -2,7 +2,7 @@
     display: flex;
     padding: $spacer;
     border-width: 0 0 0 3px;
-    color: $gray-800;
+    color: gray("800");
     margin-bottom: $spacer;
 
     &.small {
@@ -45,20 +45,20 @@
     }
 
     &.alert-info {
-      border-left-color: $info;
+      border-left-color: theme-color("info");
       background-color: $info-light;
-      fill: $info;
+      fill: theme-color("info");
     }
 
     &.alert-danger {
-      border-left-color: $danger;
+      border-left-color: theme-color("danger");
       background-color: $danger-light;
-      fill: $danger;
+      fill: theme-color("danger");
     }
 
     &.alert-warning {
-      border-left-color: $warning;
+      border-left-color: theme-color("warning");
       background-color: $warning-light;
-      fill: $warning;
+      fill: theme-color("warning");
     }
   }
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