nth-child with mod (or modulo) operator

No, :nth-child() only supports addition, subtraction and coefficient multiplication.

I gather you're trying to pick up the first 6 elements (as n mod 7 for any positive integer n only gives you 0 to 6). For that, you can use this formula instead:

:nth-child(-n+6)

By negating n, element counting is done backwards starting from zero, so these elements will be selected:

 0 + 6 = 6
-1 + 6 = 5
-2 + 6 = 4
-3 + 6 = 3
-4 + 6 = 2
-5 + 6 = 1
...

jsFiddle demo

If you want to use nth-child with modulo k, just specify:

nth-child(kn)

For example, if you want to specify style for 3, 6, 9, .. elements, just specify (k = 3), and use:

nth-child(3n)

You can also specify an offset:

nth-child(kn+offset)