I had a similar issue and I just overrode the row background with backgrounds on the TD's inside of them. Depending on your desired outcome, this may work for you too?
tr:nth-child(odd) {
background: #DDE;
}
tr:nth-child(odd) td[rowspan] {
background: #FFF;
}
Of course you can override other rows like headers, etc with a class or th.