Fix mixins import

src/core/public/index.scss

@@ -1,14 +1,10 @@
-// Functions need to be first, since we use them in our variables and mixin definitions
-@import '@elastic/eui/src/global_styling/functions/index';
-
-// Variables come next, and are used in some mixins
-@import '@elastic/eui/src/global_styling/variables/index';
-
-// Mixins provide generic code expansion through helpers
-@import '@elastic/eui/src/global_styling/mixins/index';
+// This file is built by both the legacy and KP build systems so we need to
+// import this explicitly
+@import '../../legacy/ui/public/styles/_styling_constants';
 
 @import './chrome/index';
 @import './overlays/index';
 @import './rendering/index';
+
 // Global styles need to be migrated
 @import '../../legacy/ui/public/styles/_legacy/_index';