Update toast drop-shadow opacity

- Created sass variable for box shadow to be reused for
  elements requiring box-shadow
- Update toast opacity to 0.3

Signed-off-by: Dixsie Wolmers <dixsie@ibm.com>
Change-Id: Icfc0a58944fd5fe6d2c677e99564910fc2c67f1f
diff --git a/src/assets/styles/bmc/custom/_toasts.scss b/src/assets/styles/bmc/custom/_toasts.scss
index 42b91db..dcc3985 100644
--- a/src/assets/styles/bmc/custom/_toasts.scss
+++ b/src/assets/styles/bmc/custom/_toasts.scss
@@ -5,6 +5,7 @@
 .toast {
   padding: $spacer/2 $spacer/2 $spacer/2 $spacer;
   border-width: 0 0 0 3px;
+  box-shadow: $box-shadow;
   .close {
     font-weight: 300;
     opacity: 1;