Перенёс примесь focus() в файл миксинов формы

src/less/mixins/focus-mixin.less

-.focus() {
-  outline: 0;
-  box-shadow: 0 0 0 2px fade(@color-main, 30%);
-}

src/less/mixins/forms-mixins.less

@@ -9,3 +9,8 @@
     display: none;
   }
 }
+
+.focus() {
+  outline: 0;
+  box-shadow: 0 0 0 2px fade(@color-main, 30%);
+}

src/less/mixins/mixins.less

 @import 'grid-mixins.less';
 @import 'btn-mixins.less';
 @import 'responsive-mixins.less';
-@import 'focus-mixin.less';
 @import 'forms-mixins.less';