angular/adev/shared-docs/styles/_split.scss

$gutter-border: 1px solid var(--senary-contrast) !important;

as-split {
  ::ng-deep .as-split-gutter {
    flex-basis: 5px !important;
    background-color: inherit !important;
    position: relative;
  }

  &.as-horizontal.docs-editor {
    ::ng-deep .as-split-gutter {
      border-inline: $gutter-border;
    }
  }

  &.as-vertical.docs-editor {
    ::ng-deep .as-split-gutter {
      border-block-start: $gutter-border;
    }
  }

  &.as-vertical.docs-right-side {
    ::ng-deep .as-split-gutter {
      border-block-start: $gutter-border;
    }
  }
}