Fix mixins/extends confusion

sass/components/modal.sass

@@ -29,7 +29,7 @@ $modal-card-body-background-color: $white !default
 $modal-card-body-padding: 20px !default
 
 .modal
-  +overlay
+  @extend %overlay
   align-items: center
   display: none
   flex-direction: column
@@ -42,7 +42,7 @@ $modal-card-body-padding: 20px !default
     display: flex
 
 .modal-background
-  +overlay
+  @extend %overlay
   background-color: $modal-background-background-color
 
 .modal-content,
@@ -59,7 +59,7 @@ $modal-card-body-padding: 20px !default
     width: $modal-content-width
 
 .modal-close
-  +delete
+  @extend %delete
   background: none
   height: $modal-close-dimensions
   position: fixed