discourse/app/assets/stylesheets/common/admin/admin_reports.scss

.admin-reports {
  .admin-report {
    width: 100%;

    .report-header {
      padding-bottom: 0.5em;
      margin-bottom: 1em;
      border-bottom: 1px solid var(--primary-low);
    }
  }

  .admin-report-chart .chart-canvas-container .chart-canvas {
    height: 400px;
  }
}