I'm not quite sure, which class is missing here? Is the class actually needed in this case for alternate colors ? Does maybe using tbody tr:nth-child(even) td { background-color: darken(@theme-color-background-tinyContrast, 10%) !important;} work as well?

Until now I've solved using this rules in my theme.less

table.dataTable tbody tr:nth-child(odd) td,
table.dataTable tbody tr.level0:nth-child(odd) td { background-color: @theme-color-background-tinyContrast !important; }
table.dataTable tbody tr.level1:nth-child(odd) td { background-color: darken(@theme-color-background-tinyContrast, 5%) !important; }
table.dataTable tbody tr.level2:nth-child(odd) td { background-color: darken(@theme-color-background-tinyContrast, 10%) !important; }
table.dataTable tbody tr.level3:nth-child(odd) td { background-color: darken(@theme-color-background-tinyContrast, 15%) !important; }
table.dataTable tbody tr.level4:nth-child(odd) td { background-color: darken(@theme-color-background-tinyContrast, 20%) !important; }
table.dataTable tbody tr.level5:nth-child(odd) td { background-color: darken(@theme-color-background-tinyContrast, 25%) !important; }

normal tr «oddity» takes @theme-color-background-tinyContrast then, going deeper in action tree datatable's rows you get 5 shades of grey 5% darker.

Rendered html by the browser draws <tr class=" ", doesn't create aesthetic problem but I don't think is strictly correct to declare empty arguments in html tags.

Thanks for letting us know. I will close the issue for now. While it's not strictly correct to declare empty arguments and causes more bandwidth etc we sometimes have these cases. It's a bit of a trade-off when it's simpler to maintain cases like this: https://github.com/piwik/piwik/blob/2.16.1/plugins/CoreVisualizations/templates/_dataTableViz_htmlTable.twig#L37