Add dark mode theme support

[SarthakB11]

Dark Theme Implementation

Description

• Introduced a comprehensive dark mode implementation and theme settings.
• Improved user experience and accessibility by allowing users to switch between light, dark, and system-default themes.

Changes I have made:

Theme Context System

• Created a new themeContext.tsx file for global theme context
• Implemented theme persistence using AsyncStorage
• Added support for three theme options: light, dark, and system-based

UI Components

• Added ThemeToggle.tsx component for easy theme switching
• Integrated toggle functionality in main screens
• Ensured proper icon changes during theme switching

Theme Configuration

• Updated tailwind.config.js with 'class' strategy for dark mode
• Added consistent color variables for theming
• Defined specific color palettes for both themes

App Integration

• Modified all UI components to respond to theme changes
• Updated all screens to support dark mode styling
• Created a dedicated Settings screen for theme management

Code Quality

• Used consistent styling conventions across the app
• Maintained theme-aware styling in all components
• Ensured smooth transitions between theme states

[SarthakB11]

@Asp-irin @JiyaGupta-cs kindly review.

[SarthakB11]

@keerthiseelan-planetread @PlanetRead kindly review.