added darktheme

[skully-coder]

Added dark theme, and fixed some minor text related errors, and made text in the radio button description selectable

[skully-coder]

Closes #9

[skully-coder]

@masteradit can you review and merge this?

[masteradit]

Hey! I am sorry, got stuck in some work in the last few days. I checked out the code and have a few suggestions.

I think it would be easier to add this in ThemeData instead of manually specifying the colour of text everywhere:

darkTheme: ThemeData(
  primarySwatch: Colors.teal,
  brightness: Brightness.dark,
),
themeMode: themeVariable, // this variable can store ThemeMode.dark, ThemeMode.light or ThemeMode.system

Now, using setState, the value of themeVariable can be switched.