I am re-wiring my bike lights to add a smartphone charger and improve the connections. Wires are going in pairs, one wire in a pair being marked with a white stripe. Since I am using LED lights, polarity is important. On electronics SE and in many places on the Internet I found that the white-marked wire should go to the plus. However, my hub dynamo has the marked wire connected to the contact marked with the ground symbol (so it is connected to the frame). Usually in electronics the ground is negative. Is it a mistake of the bike manufacturer or am I missing something?
No mistake - there is no standard. 50% of the time the strip is -ve, 50% positive.
Get a multimeter - $10 for a cheapy, and remove any guess work.........
White stripe is ground. Consistency is a virtue, and Schmidt's obvious decision tells us the industry default. In principle, the two conductors are interchangeable, because AC. However, in the reality of bicycles, there are practical considerations. Some dynamos ground one side of the circuit through the axle and some don't. Some lights ground one side through the mounting screws and some don't. Some bike frames are conductive and some aren't (and headset bearings are intermittent). So it is important for all deliberate connections to have compatible ideas of what is ground. Otherwise you have a short circuit. When a mechanic replaces your dynamo or light with a unit that makes an assumption about what's grounded, the assumption better match your wiring.
cheap dynamos create AC (which also works for LEDs) you can test that easily by reversing the polarity of the LEDs and see if they light up in both orientations
just add a diode (or diode bridge) to the connection for the charger to go from AC to DC