You can set any image as a custom background for the editor and all tool windows in JetBrains Rider.
Open the Settings/Preferences dialog Ctrl+Alt+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.