Router>getSceenStyle does not repull when switching scenes light/dark mode

### Version
Tell us which versions you are using:

- react-native-router-flux v4.2.0
- react v16.13.1
- react-native v0.63.4

### Expected behaviour
Background would not flash when changing scenes

### Actual behaviour

### Steps to reproduce
For non-obvious bugs, please fork this component, modify Example project to reproduce your issue and include link here.
1. setup a couple of scenes, make the component have a background color of black/white, depending on dark/light mode
2. set the router's sceneStyle or getSceneStyle to display the same background
3. open the app and toggle between scenes. Notice that the background is smooth. Now, with the app open, switch between light/dark mode and toggle between scenes. Notice that the background flashes quickly from the default color.

### Reproducible Demo

Please provide a minimized reproducible demonstration of the problem you're reporting.

Issues that come with minimal repro's are resolved much more quickly than issues where a maintainer has to reproduce themselves.