Use the individual background properties instead of the shorthand.

[ci skip]

docs/assets/scss/_ads.scss

@@ -13,7 +13,7 @@
   line-height: 1rem !important;
   color: $bd-purple-light !important;
   text-align: left;
-  background: darken($bd-purple, 10%) !important;
+  background-color: darken($bd-purple, 10%) !important;
   border: 0 !important;
 }
 

docs/assets/scss/_masthead.scss

@@ -3,8 +3,8 @@
   padding: 3rem ($grid-gutter-width / 2) 2rem;
   color: $bd-purple-light;
   text-align: center;
-  background: -webkit-linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
-  background:         linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
+  background-image: -webkit-linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
+  background-image:         linear-gradient(135deg, darken($bd-purple, 20%), $bd-purple, lighten(saturate($bd-purple, 5%), 15%));
 
   .bd-booticon {
     margin: 0 auto 2rem;