In the v11 Modern Designer, we're using this CSS to alternate the color of each table row (light purple and dark purple:

// change table button color (alternate rows)
.costingTable tr:nth-child(even) .lf-icon { color: rgb(255, 255, 255); }

// alternate table tr background colors - odd
.costingTable tr:nth-child(odd) { background-color: rgb(227, 219, 236); }

// alternate table tr background colors - even
.costingTable tr:nth-child(even) { 
  background-color: rgba(73, 43, 116, 0.6);
  color: initial;
}

// alternate table tr background colors - even - currency unit
.costingTable tr:nth-child(even) .currency-unit { 
  color: white;
}

 .costingTable .fl-component input:not([type="checkbox"]):not([type="radio"])[readonly], .fl-component textarea[readonly] {
  background-color: #f5f2f8 !important; 
}

Also worth noting is that you can apply classes using Field Rules in v11, so if you need to only change the row color conditionally, you could apply the CSS class based on a field value (for example).