Combination of Automattic's _s theme and Bootstrap 4 by holger1411.
http://understrap.com
Frameworks · WordPress Theme
Get access to data on 19,522 websites that are UnderStrap Customers. We know of 8,434 live websites using UnderStrap and an additional 11,088 sites that used UnderStrap historically and 4,103 websites in the United States.
Get a list of 19,522 websites using UnderStrap which includes location information, hosting data, contact details, 8,434 currently live websites and an additional 5,986 domains that redirect to sites in this list. 11,088 sites that used this technology previously and 4,103 websites in the United States currently using UnderStrap.