You can easily find out the CSS used by reading the following guide. It will help you in the future
https://meta.discourse.org/t/make-css-changes-on-your-site/168101#browser-inspection-tools-7
The customization uses SCSS nesting, but the compiled CSS is:
.above-main-container-outlet.welcome-link-banner-connectors .welcome-wrapper .featured-banner-link>div:nth-of-type(2),
.below-site-header-outlet.welcome-link-banner-connector .welcome-wrapper .featured-banner-link>div:nth-of-type(2) {
border-left: 1px solid var(--primary-low);
border-right: 1px solid var(--primary-low);
}