Locate the line:
https://github.com/magento/magento2/blob/22629abe10f9c640fe47b6081d57fedccdea8e0f/app/design/frontend/Magento/luma/web/css/source/_theme.less#L13
It is empty.
Place here the code:
@import 'components/_modals.less';
It fixes the problems:
https://github.com/magento/magento2/issues/2273
https://github.com/magento/magento2/issues/2286