see panelGrid columnClasses property
tld
doc said:
Comma-delimited list of CSS style classes that will be applied to
the columns of this table. A space separated list of classes may
also be specified for any individual column. If the number of
elements in this list is less than the number of columns specified
in the "columns" attribute, no "class" attribute is output for each
column greater than the number of elements in the list. If the
number of elements in the list is greater than the number of
columns specified in the "columns" attribute, the elements at the
posisiton in the list after the value of the "columns" attribute are
ignored.
The default column classes would be rendered as : icePnlGrdCol1,
icePnlGrdCol2 alternatively