Set background to even rows
Style table tr:nth-child(even) is setting style to every even row. If you want to set style to odd rows you should use nth-child(odd).
<!DOCTYPE html>
<html>
<head>
<style>
table {
background-color:lightgreen;
}
table tr:nth-child(even) {
background-color:lightgreen;
}
table tr:nth-child(odd) {
background-color:green;
}
</style>
<meta charset="UTF-8">
</head>
<body>
<table>
<tr>
<th>First H</th>
<th>Second H</th>
<th>Third H</th>
</tr>
<tr>
<td>cell </td>
<td>cell </td>
<td>cell </td>
</tr>
<tr>
<td>cell </td>
<td>cell </td>
<td>cell </td>
</tr>
<tr>
<td>cell </td>
<td>cell </td>
<td>cell </td>
</tr>
</table>
</body>
</html>