Fix mixins/extends confusion

sass/components/breadcrumb.sass

@@ -8,8 +8,8 @@ $breadcrumb-item-padding-horizontal: 0.75em !default
 $breadcrumb-item-separator-color: $grey-light !default
 
 .breadcrumb
-  +block
-  +unselectable
+  @extend %block
+  @extend %unselectable
   font-size: $size-normal
   white-space: nowrap
   a

sass/components/level.sass

@@ -1,5 +1,5 @@
 .level
-  +block
+  @extend %block
   align-items: center
   justify-content: space-between
   code

sass/components/message.sass

@@ -19,7 +19,7 @@ $message-body-pre-code-background-color: transparent !default
 $message-header-body-border-width: 0 !default
 
 .message
-  +block
+  @extend %block
   background-color: $message-background-color
   border-radius: $message-radius
   font-size: $size-normal

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

sass/components/navbar.sass

@@ -201,7 +201,8 @@ a.navbar-item,
 .navbar-link:not(.is-arrowless)
   padding-right: 2.5em
   &::after
-    +arrow($navbar-dropdown-arrow)
+    @extend %arrow
+    border-color: $navbar-dropdown-arrow
     margin-top: -0.375em
     right: 1.125em
 

sass/components/pagination.sass

@@ -54,7 +54,7 @@ $pagination-shadow-inset: inset 0 1px 2px rgba($black, 0.2)
 .pagination-link,
 .pagination-ellipsis
   +control
-  +unselectable
+  @extend %unselectable
   font-size: 1em
   padding-left: 0.5em
   padding-right: 0.5em

sass/components/tabs.sass

@@ -27,9 +27,9 @@ $tabs-toggle-link-active-border-color: $link !default
 $tabs-toggle-link-active-color: $link-invert !default
 
 .tabs
-  +block
+  @extend %block
   +overflow-touch
-  +unselectable
+  @extend %unselectable
   align-items: stretch
   display: flex
   font-size: $size-normal