Feat/#120 dark mode

[umangutkarsh]

Brief Title: Added Dark-Theme functionality

Acceptance Criteria fulfillment

• [X] Added Dark-Theme feature

Fixes #120

Methodology

• Added a DarkTheme object like the DefaultTheme in packages/react/src/theme/DarkTheme.js
• Created a themeStore, just like other stores.
• Incorporated DarkTheme in Buttons, Menu, Icons, and in stories
• Added button in ChatHeader.js to toggle theme
• Wrapped the app using ThemeProvider in index.js

Video/Screenshots

[umangutkarsh]

Please LMN if it's working or not, or if anything else needs to be done. Or another methodology needs to be followed.