Every cell should have a css class (.rf-cst-c
it is in the actual richfaces showcase), so you could do something like:
.rf-cst-c:nth-child(odd) td {
background: #000;
}
.rf-cst-c:nth-child(even) td {
background: #5b5b5b;
}
.rf-cst-c:nth-child(3n) td {
background: #fff;
}
If the css class is changing due to different versions of rich faces, it should be no problem to use your browser's debugging tools and get hold of the css class in use.