You can set any image as a custom background for the editor and all tool windows in AppCode.
Open the Preferences dialog ⌃ ⌥ S, select , and click the Background Image button.
In the Background Image dialog, specify the image you want to use as the background, its opacity, filling and placement options. If necessary, mirror the image vertically or horizontally. You can set separate images for the editor and tool windows, and for the empty frame (when no files are opened in the editor).
If you want to set the image only for the current project, select the This project only checkbox.
Click OK to apply the changes.
If you open an image in the editor or add it to the Project tool window, you can right-click the image and click Set Background Image in the context menu. You can also use the Find Action command ⌃ ⇧ A to execute the Set Background Image action.