Virtually every color in Grid is customizable, with clear names and descriptions in the theme settings. The colors are organized into five categories: Background, General, Buttons, Header, and Dropdowns/Table headers.
For Background, you have the option to upload a Background image. A high-resolution image is ideal for this option since the image will be spread across a large area.
Click on the swatch to edit the colors. A popup with a color selector will appear, which allows you to adjust and experiment with custom colors.