:root {
  /* --- Foundation Colors: Light Theme (Default) --- */
  --color-bg-body: #ffffff;
  --color-bg-surface: #f8f9fa;
  --color-text-primary: #1a202c;
  --color-text-secondary: #4a5568;
  --color-text-tertiary: #718096;

  /* --- Accent Colors --- */
  --color-accent: #2563eb;
  --color-accent-light: #3b82f6;
  --color-accent-dark: #1d4ed8;
  --color-accent-hover: #1e40af;
  --color-accent-subtle: #e0e7ff;
  /* lighter, friendly for backgrounds */

  /* --- Button Colors --- */
  --btn-bg: #e8eafc;
  /* Soft blue/gray for buttons */
  --btn-bg-hover: #2563eb;
  /* Accent blue on hover */
  --btn-text: #1a202c;
  --btn-text-hover: #fff;
  --btn-border: #b5bedc;

  /* --- Gradients --- */
  --gradient-primary: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  --gradient-accent: linear-gradient(135deg, #2563eb 0%, #7c3aed 100%);
  --gradient-hero: linear-gradient(135deg, rgba(37, 99, 235, 0.9) 0%, rgba(124, 58, 237, 0.9) 100%);
  --gradient-card: linear-gradient(145deg, #f8f9fa 0%, #ffffff 100%);

  /* --- Semantic Colors --- */
  --color-success: #10b981;
  --color-warning: #f59e0b;
  --color-info: #3b82f6;
  --color-error: #ef4444;

  /* --- Borders & Shadows --- */
  --color-border: #e5e7eb;
  --color-border-subtle: #f3f4f6;
  --shadow-sm: 0 1.5px 3px 0 rgba(0, 0, 0, 0.06);
  --shadow-md: 0 4px 8px -2px rgba(0, 0, 0, 0.10);
  --shadow-lg: 0 10px 30px rgba(0, 0, 0, 0.12);
  --shadow-xl: 0 20px 55px -7px rgba(0, 0, 0, 0.13);

  /* --- Typography --- */
  --font-sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
  --font-serif: "Georgia", "Times New Roman", Times, serif;

  --font-weight-normal: 400;
  --font-weight-medium: 500;
  --font-weight-bold: 600;
  --font-weight-heavy: 700;
  --font-weight-black: 800;

  --line-height-base: 1.6;
  --line-height-heading: 1.25;

  /* --- Spacing System --- */
  --space-2xs: 0.25rem;
  --space-xs: 0.5rem;
  --space-sm: 0.75rem;
  --space-md: 1.5rem;
  --space-lg: 3rem;
  --space-xl: 5rem;

  /* --- Layout --- */
  --max-width: 900px;
  --header-height: 70px;
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-xl: 24px;

  /* --- Transitions --- */
  --transition-fast: 0.2s ease;
  --transition-medium: 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
  --transition-slow: 0.5s cubic-bezier(0.4, 0, 0.2, 1);
}

/* --- Dark Theme --- */
[data-theme="dark"] {
  --color-bg-body: #0f172a;
  --color-bg-surface: #1e293b;
  --color-text-primary: #f1f5f9;
  --color-text-secondary: #cbd5e1;
  --color-text-tertiary: #94a3b8;

  /* --- Accent Colors, tweaked for dark background --- */
  --color-accent: #60a5fa;
  /* defined blue */
  --color-accent-light: #93c5fd;
  --color-accent-dark: #2563eb;
  --color-accent-hover: #2563eb;
  --color-accent-subtle: #1e293b;
  /* deep background blue */

  /* --- Button Colors --- */
  --btn-bg: #283042;
  /* deep navy/blue for dark mode buttons */
  --btn-bg-hover: #60a5fa;
  --btn-text: #f1f5f9;
  --btn-text-hover: #1e293b;
  --btn-border: #334155;

  /* --- Gradients --- */
  --gradient-primary: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  --gradient-accent: linear-gradient(135deg, #60a5fa 0%, #a78bfa 100%);
  --gradient-hero: linear-gradient(135deg, rgba(96, 165, 250, 0.9) 0%, rgba(167, 139, 250, 0.9) 100%);
  --gradient-card: linear-gradient(145deg, #1e293b 0%, #0f172a 100%);

  /* --- Borders & Shadows --- */
  --color-border: #334155;
  --color-border-subtle: #1e293b;
  --shadow-sm: 0 1.5px 3px 0 rgba(0, 0, 0, 0.19);
  --shadow-md: 0 4px 8px -2px rgba(0, 0, 0, 0.24);
  --shadow-lg: 0 10px 30px rgba(0, 0, 0, 0.23);
  --shadow-xl: 0 20px 55px -7px rgba(0, 0, 0, 0.29);
}