@salesforce-ux/design-system/css/regions/base/index.css

Salesforce Lightning Design System

/*! Lightning Design System 2.12.0 */
@charset "UTF-8";
.slds-region {
  background-color: white;
}
.slds-region__pinned-top {
  background-color: white;
  border-bottom: 1px solid #dddbda;
}
.slds-region__pinned-left {
  background-color: white;
  border-right: 1px solid #dddbda;
}
.slds-region__pinned-right {
  background-color: white;
  border-left: 1px solid #dddbda;
}