onecol.css

/*
 * @file
 * Provides the layout styles for layout_onecol.
 */
.layout--onecol .layout__region {
  width: 100%;
}

File

core/modules/layout_discovery/layouts/onecol/onecol.css

/*
 * @file
 * Provides the layout styles for layout_onecol.
 */
.layout--onecol .layout__region {
  width: 100%;
}