An electronic device may have input-output devices such as buttons formed from capacitive touch sensor electrodes. A transparent layer may be provided with opaque masking structures having a symbol-shaped opening aligned with a capacitive touch sensor electrode for a button or other input device. The symbol-shaped opening may be adjacent to the electrode and may be illuminated with light from a lighting system. The lighting system may include a light guide plate. The light guide plate may have edges that are oriented at non-zero angle with respect to each other. Light-emitting diodes of different colors may emit light into the edges. The light may be extracted from the light guide plate using bumps in a light extraction region. Light may be collimated using ribs formed on an opposing side of the light guide plate.