[V4] fix themes related examples

[Simek]

This PR fixes small issues in themes related examples :

• missing theme variable initialization in static context usage example
• missing context initialization in "Themes inside navigationOptions"
• missing ThemeContext import in "Theming tabs and other similar navigators"

[WoLewicki]

Thanks for contribution! I think we should add ThemeConstants to "Themes inside navigationOptions" to be consistent and don't import ThemeContext in "Theming tabs and other similar navigators" but rather create it as in "Themes inside navigationOptions". Could you do it? And what do you think @satya164 ?

[satya164]

User needs to import ThemeContext to be able to use the theme from React Navigation. Creating own theme context means it's a different theme system. I think user shouldn't create separate context if they just need to integrate with React Navigation's themes.

[WoLewicki]

Yes, but in the examples, there are used custom values from ThemeConstants, like this
let currentTheme = ThemeConstants[screenProps.theme];

[satya164]

I'm not following. I mean current ThemeConstants usage is ok, but the ThemeContext should be imported instead of being created by the user.

[WoLewicki]

Ok, so just change the creation of ThemeContext to importing it from react-navigation ?

[satya164]

Yeah

[Simek]

@WoLewicki @satya164 Thank you for the comments.

I have modified the PR according to your discussion. I hope I have understood the required change properly.

[WoLewicki]

Thanks for the update! I think it is okay now, if @satya164 thinks so too, we can merge it.