prevent warnings during assets build process, re #2801

resources/assets/stylesheets/scss/courseware/layouts/ribbon.scss

@@ -408,7 +408,7 @@ $consum_ribbon_width: calc(100% - 58px);
 
         &.contentbar-button-menu,
         &.cw-ribbon-button-menu {
-            @include background-icon(table-of-contents, clickable, 24);
+            @include background-icon(table-of-contents, $size: 24px);
         }
 
         &.contentbar-button-zoom::before {

resources/assets/stylesheets/scss/table_of_contents.scss

@@ -47,7 +47,7 @@ ul.numberedchapters {
         right: 0;
         top: 12px;
         cursor: pointer;
-        @include background-icon(decline, clickable, 24);
+        @include background-icon(decline, $size: 24px);
         background-repeat: no-repeat;
         background-size: 24px;
         background-position: center right;