Combination of Automattic's _s theme and Bootstrap 4 by holger1411.
http://understrap.com
Frameworks · WordPress Theme
Get access to data on 17,137 websites that are UnderStrap Customers. We know of 8,430 live websites using UnderStrap and an additional 8,707 sites that used UnderStrap historically and 4,120 websites in the United States.
Get a list of 17,137 websites using UnderStrap which includes location information, hosting data, contact details, 8,430 currently live websites and an additional 7,413 domains that redirect to sites in this list. 8,707 sites that used this technology previously and 4,120 websites in the United States currently using UnderStrap.