You can add your own deskmat design to the deskmat by editing three materials, Deskmat_Mat, Deskmat_Edging and Deskmat_Stitching. You can find these in the Material Properties panel.
Find the image texture node in each of the materials and load your own image into the node.
By default, the deskmat is able to scale to 1mx1m so your design should be positioned on a 1mx1m square.
If you prefer to use another size image, make sure to set the position in the mapping node to half the deskmat size and the scale to the size in meters.
Image: Milky Way by metalbender and the spacey
You can also replace the image texture node with a pattern.
