Since Chrome version 79 you can toggle between prefers-color-scheme: dark and prefers-color-scheme: light from the Rendering panel
- Open Developer tools (otherwise the key combination below opens the print dialog)
- Open the Command Control: Ctrl+Shift+P or Command+Shift+P (Mac)
- Type “Show rendering”
- Set the Emulate CSS media feature prefers-color-scheme to the value you want to debug
