1- In Windows: Make an image of 1 pixel in Paint;
2- In any program/web service that can do this: Apply transparency to this referred image (You can use the program Gif Movie Gear or the web server Zamba, as my suggestion);
3- In Construct 2: Insert an "Tiled Background"; load your transparent image for this tiled background, resize your "Tiled Background" and paste in laterals; attach the "Solid" behavior to this tiled background.
Note: The ball will need to have the "Solid" behavior, too. Good luck