Just click on the green flag to generate a space background. You can use left and right arrows to change the location settings (choosing from a limited selection of scratch sprite). When you are done, you can right click on the stage to save a background/sprite ready to be uploaded in a Scratch project
I use the random block in almost all my project and I felt uneasy working on this project. I had to force me to stop procrastinating and think on what to do (like on the day one project). I felt like cheating on this challenge. I had to create something useful (I hope) to overcome that sensation and I made a space themed background creator. I reused several sprites and images from the scratch library, in order to generate images that seems familiar to the scratch users.