Add Custom Theme to storybook

[ssamaresh]

GOAL

Now that we have storybook implemented for the shared components, we want to build up on it and create a custom theme to improve the UI (styling) for it.

Refer: Storybook Theming

This will entail creating a CSS theme (colors, typography, e.t.c) for the storybook UI.
Possibly one that can also eventually be shared with the Awesome learning UI.