Improve dark mode consistency, accessibility, and visual hierarchy

[Keerthan-Bhat]

Description
Overview This PR improves the core dark mode experience of the application by introducing a more cohesive color palette, improving text-to-background contrast, and establishing a clearer elevation hierarchy across components.

Changes Made

Theme Variables (index.css):
Refined --color-background to a deeper dark tone (#090B10) to reduce glare and improve contrast.
Updated surface variables (--color-surface, --color-surface-elevated) for better visual depth.
Softened --color-text-primary to #F8FAFC to reduce eye strain while maintaining accessibility.
Introduced a new semantic variable --color-border-subtle (rgba(255, 255, 255, 0.06)) to replace hardcoded border opacities.
Component Refactoring:
BookPreviewCard.jsx: Replaced hardcoded utility classes (border-white/10, text-white) with the new semantic theme variables (border-border-subtle, text-text-primary) for consistent styling and easier future theming.
ActivityFeed.jsx: Replaced hardcoded utility classes (border-white/5, text-white) with semantic variables, ensuring a unified look across the dashboard.
Impact These changes give the application a more premium and polished feel, make the codebase easier to maintain by utilizing centralized CSS variables, and significantly improve reading comfort for users in dark environments.

[AnirbansarkarS]

@Keerthan-Bhat whaat things you changed mark in sshot

[Keerthan-Bhat]

1. Global Color Palette & Theme Variables (index.css)
   I replaced generic colors with a carefully curated, premium dark theme palette using Tailwind CSS variables:

Core Background: Changed from a standard dark gray to a deep, atmospheric black/navy (#090B10).
Surface Colors: Cards and containers use a slightly lighter shade (#141724) to create depth and separate content from the background.
Primary Accent: A vibrant "Racing Red" (#ff002b) is used for active states, indicators, and primary buttons.
Borders: Subtle borders are styled with a translucent white (rgba(255, 255, 255, 0.06)), making lines clean without being distracting.
2. Typography & Readability
Primary Text: Set to an off-white color (#F8FAFC) to reduce eye strain while maintaining high contrast.
Secondary/Muted Text: Used shades like #94A3B8 and #64748B for descriptions, labels, and timestamps to create a clear visual hierarchy.
3. Component-Specific Changes (e.g., BookPreviewCard, ActivityFeed)
Semantic Classes: Hardcoded color classes (like bg-zinc-900 or text-gray-200) were replaced with semantic classes like bg-surface and text-text-primary. This ensures that if the theme variables are updated in the future, all components will update automatically.
Button Effects: The "Save to Library" and other primary buttons received a custom glowing effect using the primary red color with a low-opacity shadow (shadow-primary/20).
4. Discover Page Specifics
The Active Filter (like the "Fiction" category button) is filled with the solid primary red.
The Sidebar uses a thin border and gradient indicators (like the red dot next to "Discover") to show the current active page without cluttering the layout.
The Banner was kept vibrant with a gradient to serve as a strong focal point on the page.
These changes combined create a modern, high-contrast, and visually immersive dark mode experience across the entire application.