Dhtmlx Grid column separator width

I have this grid with 2line header:

and I would like to have more white space between the months (highlighted with orange). Now I’m doing it with .setCellTextStyle() by increasing the border-width for each cell where it is needed. The problem with this approach is, that every 3rd column’s width will be decreased. Is it possible to do this in a more elegant way?

Unfortunately the implemented borders solution is the only available one.