feat(tokens): introduce foundational CSS design token system (Part 1 of #6606) (#7315)

* feat: add centralized CSS design tokens system

Introduces tokens.css with semantic variables for colors, spacing, typography, and shadows. Includes support for light, dark, and high-contrast themes.

* Added the minimal import @import url in tokens.css

Author: unmeshgb

css/style.css

@@ -1,3 +1,5 @@
+@import url("tokens.css");
+
 html, body {
   overscroll-behavior-x: none;
 }

css/tokens.css

@@ -0,0 +1,148 @@
+/**
+ * MusicBlocks Design Tokens
+ * A centralized design system for consistent UI theming.
+ */
+
+:root {
+  /* =========================================================================
+     Base Colors (Light Theme Default)
+     ========================================================================= */
+  --color-bg-primary: #ffffff;
+  --color-bg-secondary: #f3f4f6;
+  --color-bg-tertiary: #e5e7eb;
+  
+  --color-text-primary: #1f2937;
+  --color-text-secondary: #4b5563;
+  --color-text-tertiary: #9ca3af;
+  --color-text-inverse: #ffffff;
+  
+  --color-border-primary: #d1d5db;
+  --color-border-secondary: #e5e7eb;
+  
+  /* Semantic Brand & State Colors */
+  --color-brand-primary: #3b82f6;
+  --color-brand-primary-hover: #2563eb;
+  --color-brand-secondary: #8b5cf6;
+  
+  --color-success: #10b981;
+  --color-success-bg: #d1fae5;
+  --color-warning: #f59e0b;
+  --color-warning-bg: #fef3c7;
+  --color-error: #ef4444;
+  --color-error-bg: #fee2e2;
+  --color-info: #3b82f6;
+  --color-info-bg: #dbeafe;
+  
+  /* =========================================================================
+     Spacing (Base 4px scale)
+     ========================================================================= */
+  --spacing-xs: 0.25rem;  /* 4px */
+  --spacing-sm: 0.5rem;   /* 8px */
+  --spacing-md: 1rem;     /* 16px */
+  --spacing-lg: 1.5rem;   /* 24px */
+  --spacing-xl: 2rem;     /* 32px */
+  --spacing-2xl: 3rem;    /* 48px */
+  
+  /* =========================================================================
+     Typography
+     ========================================================================= */
+  --font-family-system: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
+  --font-family-mono: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
+  
+  --font-size-xs: 0.75rem;   /* 12px */
+  --font-size-sm: 0.875rem;  /* 14px */
+  --font-size-md: 1rem;      /* 16px */
+  --font-size-lg: 1.125rem;  /* 18px */
+  --font-size-xl: 1.25rem;   /* 20px */
+  --font-size-2xl: 1.5rem;   /* 24px */
+  
+  --font-weight-normal: 400;
+  --font-weight-medium: 500;
+  --font-weight-semibold: 600;
+  --font-weight-bold: 700;
+  
+  --line-height-tight: 1.25;
+  --line-height-normal: 1.5;
+  --line-height-loose: 2;
+  
+  /* =========================================================================
+     Border Radius
+     ========================================================================= */
+  --radius-sm: 0.25rem;   /* 4px */
+  --radius-md: 0.375rem;  /* 6px */
+  --radius-lg: 0.5rem;    /* 8px */
+  --radius-xl: 0.75rem;   /* 12px */
+  --radius-full: 9999px;  /* Pill shape */
+  
+  /* =========================================================================
+     Shadows
+     ========================================================================= */
+  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
+  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
+  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
+  --shadow-focus: 0 0 0 3px rgba(59, 130, 246, 0.5);
+}
+
+/* ==========================================================================
+   Dark Theme
+   ========================================================================== */
+[data-theme="dark"] {
+  --color-bg-primary: #111827;
+  --color-bg-secondary: #1f2937;
+  --color-bg-tertiary: #374151;
+  
+  --color-text-primary: #f9fafb;
+  --color-text-secondary: #d1d5db;
+  --color-text-tertiary: #9ca3af;
+  --color-text-inverse: #111827;
+  
+  --color-border-primary: #4b5563;
+  --color-border-secondary: #374151;
+  
+  --color-brand-primary: #60a5fa;
+  --color-brand-primary-hover: #3b82f6;
+  --color-brand-secondary: #a78bfa;
+  
+  /* Adjusted state backgrounds for dark mode */
+  --color-success-bg: rgba(16, 185, 129, 0.2);
+  --color-warning-bg: rgba(245, 158, 11, 0.2);
+  --color-error-bg: rgba(239, 68, 68, 0.2);
+  --color-info-bg: rgba(59, 130, 246, 0.2);
+  
+  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.4);
+  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.5), 0 2px 4px -1px rgba(0, 0, 0, 0.4);
+  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.5), 0 4px 6px -2px rgba(0, 0, 0, 0.4);
+  --shadow-focus: 0 0 0 3px rgba(96, 165, 250, 0.5);
+}
+
+/* ==========================================================================
+   High-Contrast Theme
+   ========================================================================== */
+[data-theme="high-contrast"] {
+  --color-bg-primary: #000000;
+  --color-bg-secondary: #000000;
+  --color-bg-tertiary: #000000;
+  
+  --color-text-primary: #ffffff;
+  --color-text-secondary: #ffff00;
+  --color-text-tertiary: #ffffff;
+  --color-text-inverse: #000000;
+  
+  --color-border-primary: #ffffff;
+  --color-border-secondary: #ffffff;
+  
+  --color-brand-primary: #ffff00;
+  --color-brand-primary-hover: #ffffff;
+  --color-brand-secondary: #00ffff;
+  
+  --color-success: #00ff00;
+  --color-success-bg: #000000;
+  --color-warning: #ffaa00;
+  --color-warning-bg: #000000;
+  --color-error: #ff0000;
+  --color-error-bg: #000000;
+  --color-info: #00ffff;
+  --color-info-bg: #000000;
+  
+  --shadow-focus: 0 0 0 4px #ffff00;
+}