How can I print the grid in landscape by CSS?

The grid is working well but it has so many columns that it can only print out completely by lanscape.

Can any nice professtional tell me which perperty I need to change in the "grid.css" file to print it in landscape? When I open it, I can not effectively read it. Or I need to create one by myself.

I am norvice and thank you so much for your help

Regards

Shen