Since the new theming documentation includes a section about the **border-radius** function and mixin, we should make them public. [border-radius function](https://github.com/IgniteUI/igniteui-theming/blob/f82a03149a0882f9ac5e851d4c9cda06c6e09bc1/sass/themes/_functions.scss#L127) [border-radius mixin](https://github.com/IgniteUI/igniteui-theming/blob/f82a03149a0882f9ac5e851d4c9cda06c6e09bc1/sass/themes/_mixins.scss#L95)