To create custom portfolio grids you have seen in theme preview you need Essential Grid plugin installed and activated (comes with the theme). Go to Appearance > Plugins if you have not already done this.
If you already have, just create new grid or use an imported style and choose portfolio items as item source. Next, insert Essential Grid element on this page and choose one of the grid layouts. More information can be found in User Manual.
Comprehensive documentation on Essential Grid can be found here: