[Discover][A11y] Add underline to search result highlights

src/platform/packages/shared/kbn-unified-data-table/src/components/data_table.scss

@@ -186,6 +186,10 @@
     // might be limited by that.
     max-width: ($euiSizeXXL * 12.5) !important;
   }
+
+  mark {
+    text-decoration: dotted underline;
+  }
 }
 
 @include euiBreakpoint('xs', 's', 'm') {

src/platform/plugins/shared/unified_doc_viewer/public/components/doc_viewer_table/table_cell_value.tsx

@@ -215,6 +215,10 @@ const componentStyles = {
       '.euiDataGridRowCell__popover &': {
         fontSize,
       },
+
+      '& mark': {
+        textDecoration: 'dotted underline',
+      },
     });
   },
   docViewerValueHighlighted: ({ euiTheme }: UseEuiTheme) =>