The :nth-child() and :nth-of-type() pseudo-classes allows you to select elements with a formula.
The syntax is :nth-child(an+b), where you replace a and b by numbers of your choice.
For instance, :nth-child(3n+1) selects the 1st, 4th, 7th etc. child.
td:nth-child(3n+1) {
/* your stuff here */
}
:nth-of-type() works the same, except that it only considers element of the given type ( in the example).