Added dark mode feature

[992manav]

fixes #4
This pull request introduces a dark mode option to the Own-AI application, addressing the need for improved user experience in various lighting conditions. The addition of dark mode will help reduce eye strain, particularly in low-light environments, and can also contribute to battery savings on OLED displays.

Proposed Implementation:

Dark Mode Toggle: Added a toggle button to the user interface, allowing users to switch between light and dark themes based on their preference.
Accessibility Compliance: Ensured that dark mode maintains proper color contrast and adheres to accessibility standards to support visually impaired users.
System Preferences Integration: Implemented functionality to respect system theme preferences (light or dark) on supported browsers and devices, automatically applying the theme based on user system settings when possible.
Changes Made:

Introduced a theme toggle button in the user interface for switching between light and dark modes.
Updated CSS styles to include dark mode with proper color contrasts and accessibility considerations.
Implemented logic to detect and apply system theme preferences.

[sujal-goswami]

Hey @992manav , could you please change the current dark colour combination? You can explore combinations from colour palettes. The current one looks too dark.