Fix book banner

docs/_sass/book.sass

@@ -13,7 +13,7 @@ $book-beige: #FFEDD7
     transform: scale(1)
 
 .bd-book-banner
-  background-color: $white
+  background-color: $primary
   position: relative
 
 .bd-book-pattern,
@@ -44,8 +44,7 @@ $book-beige: #FFEDD7
     display: block
 
 .bd-book-content
-  background-color: $white
-  box-shadow: 0 40px 40px -20px rgba($black, 0.1)
+  @extend %bd-box
   max-width: 520px
   padding: 3rem
 

docs/_sass/footer.sass

@@ -29,10 +29,7 @@
   margin-top: 1.5rem
 
 %bd-footer-box
-  background-color: $white
-  border-radius: $radius-large
-  box-shadow: 0 1.5rem 1.5rem -1.25rem rgba($black, 0.1)
-  display: block
+  @extend %bd-box
   text-align: center
   .bd-footer-title
     font-size: 1.5rem