how to declare strip extern to be usuable in all files about neopixelbus HOT 2 CLOSED

If you search online this is common problem to hit, and sometimes its hard to see the tree for the forest.
Your problem is in this code...

 extern ESP8266WebServer server;
  extern NeoPixelBus<NeoRgbFeature, NeoEsp8266BitBang800KbpsMethod> rgb_led();

One line is correct, one is not. You have () on the rgb_led so the compiler thinks its a function rather than a class. remove the ().
It should look like...

 extern ESP8266WebServer server;
  extern NeoPixelBus<NeoRgbFeature, NeoEsp8266BitBang800KbpsMethod> rgb_led;

I would also suggest that you typedef the NeoPixelBus so that if you want to change it, you change it in one spot. Further makes it easier to read code. So in main.h

typedef NeoPixelBus<NeoRgbFeature, NeoEsp8266BitBang800KbpsMethod> MyPixelBus;

extern MyPixelBus rgb_led;

from neopixelbus.

Works like a charm, thanks, and yes typedef is much better ;-)

from neopixelbus.