chrome/browser/resources/side_panel/shared/sp_footer.html

<style>
  :host {
    display: flex;
    justify-content: center;
  }

  :host([pinned]) {
    border-top: var(--cr-hairline);
    box-sizing: border-box;
    display: block;
    flex-shrink: 0;
    height: 48px;
    margin-block-start: auto;
    padding: 8px;
    position: relative;
  }

  :host-context([chrome-refresh-2023]):host([pinned]) {
    border-top: 0;
    height: 56px;
  }
</style>
<slot></slot>