[SUGGESTION]: Support for darkmode

[wizlif]

Currently the app theming is using manually set colours from constants.dart, meaning the app looks the same both in light mode and dark mode. This is a bad practice as it doesn't use flutter's inbuilt theming through Theme.of(context).

We can migrate coloring to base on Flutter's theming by using the inbuilt colorSchemes to pass colours around and retire use of colours in contants.dart. This would also come in handy incase we need to support dark mode.

We wouldn't necessarily use FlutterColorSchemes but setup our own colour schemes.