From inside the Editor of your Landing Page look for “Page Settings” and scroll down to where it says “Background”. Once there select the first element called “Color” and you’ll immediately see that you can change the color of the background of your Landing Page by easily selecting the color you want.
So, just select the perfect one for your Landing Page aesthetic, even choose the opacity (level of transparency, it’s perfect to create a "reflective" or "see through" kind of effect) of the color and fine tuning it.
However, if you want to include an image as a background you can do that as well. In the background section, you’ll see a field entitled “Image”. Click right there and you’ll be able to see your Image Gallery and select the image you want or even upload a new one from your computer.
Also if you have your image hosted in your own site or blog, you can include the URL, and you'll be able to see your image as your Landing Page background.
We recommend that you use the following sizing for your reference: 1366 x 768 Pixels and the maximum weight of the image should not exceed 500 kb.
Now, once you've selected the image you want you can also choose how to use that image. You can choose the position (Left, Center or Right) , the tiling (Horizontal, Vertical or both) or even make your background image to stretch to the edges of the template canvas.