Change to use govuk_frontend_toolkit box-sizing mixin
Change to use govuk_frontend_toolkit box-sizing mixin

as per @alextea's comment
https://github.com/alphagov/static/commit/35c97f054f45f48cbbcc6e93f735cf
b3a263a51d#commitcomment-3264149

robinwhittleton/static,kalleth/static,kalleth/static,alphagov/static,alphagov/static,kalleth/static,robinwhittleton/static,tadast/static,kalleth/static,tadast/static,robinwhittleton/static,robinwhittleton/static,alphagov/static,tadast/static,tadast/static