feat: add dark mode support via prefers-color-scheme

Add automatic dark mode support that respects operating system preferences
using the `@media (prefers-color-scheme: dark)` CSS media query.

Changes:
- verso-vars.css: Add new CSS custom properties for backgrounds, borders,
  links, shadows, and message colors with dark mode variants
- code.css: Update API docs styling to use CSS variables
- search-box.css/search-highlight.css: Update search UI to use CSS variables
- Style.lean: Update manual page styling to use CSS variables

The color scheme is harmonized with lean-lang.org:
- Light: #ffffff background, #333 text, #386ee0 links
- Dark: #1e1e1e background, #eee text, #3b94ff links

Closes #641

Author: alok

src/verso-html/code.css

@@ -15,7 +15,8 @@
 body {
     font-family: var(--verso-text-font-family);
     line-height: 1.6;
-    color: #333;
+    color: var(--verso-text-color, #333);
+    background-color: var(--verso-background-color, #fff);
     height: 100vh;
     overflow: hidden;
 }
@@ -29,8 +30,8 @@ body {
 /* Sidebar */
 .sidebar {
     width: 300px;
-    background: #f8f9fa;
-    border-right: 1px solid #e9ecef;
+    background: var(--verso-surface-color, #f8f9fa);
+    border-right: 1px solid var(--verso-border-color, #e9ecef);
     overflow-y: auto;
     flex-shrink: 0;
 }
@@ -73,11 +74,11 @@ body {
 }
 
 .module-tree summary:hover {
-    background-color: #e9ecef;
+    background-color: var(--verso-border-color, #e9ecef);
 }
 
 .module-tree summary a {
-    color: #0066cc;
+    color: var(--verso-link-color, #0066cc);
     text-decoration: none;
 }
 
@@ -86,12 +87,12 @@ body {
 }
 
 .module-tree summary.current {
-    background-color: #0066cc;
-    color: white;
+    background-color: var(--verso-link-color, #0066cc);
+    color: var(--verso-background-color, white);
 }
 
 .module-tree summary.current a {
-    color: white;
+    color: var(--verso-background-color, white);
 }
 
 .module-tree summary::before {
@@ -100,7 +101,7 @@ body {
     left: -0.75rem;
     transition: transform 0.2s ease;
     font-size: 0.75rem;
-    color: #6c757d;
+    color: var(--verso-text-color-light, #6c757d);
 }
 
 .module-tree details[open] > summary::before {
@@ -114,7 +115,7 @@ body {
 }
 
 .module-tree .leaf a {
-    color: #0066cc;
+    color: var(--verso-link-color, #0066cc);
     text-decoration: none;
 }
 
@@ -123,12 +124,12 @@ body {
 }
 
 .module-tree .current {
-    background-color: #0066cc;
-    color: white;
+    background-color: var(--verso-link-color, #0066cc);
+    color: var(--verso-background-color, white);
 }
 
 .module-tree .current a {
-    color: white;
+    color: var(--verso-background-color, white);
 }
 
 /* Main content area */
@@ -141,8 +142,8 @@ body {
 
 /* Title bar */
 .title-bar {
-    background: #fff;
-    border-bottom: 1px solid #e9ecef;
+    background: var(--verso-background-color, #fff);
+    border-bottom: 1px solid var(--verso-border-color, #e9ecef);
     padding: 1rem 1.5rem;
     flex-shrink: 0;
     position: relative;
@@ -165,26 +166,26 @@ body {
 .breadcrumbs li:not(:last-child)::after {
     content: "·";
     margin: 0;
-    color: #6c757d;
+    color: var(--verso-text-color-light, #6c757d);
     font-weight: bold;
 }
 
 .breadcrumbs a {
-    color: #0066cc;
+    color: var(--verso-link-color, #0066cc);
     text-decoration: none;
     padding: 0.25rem 0.5rem;
     border-radius: 0.25rem;
     transition: background-color 0.2s ease;
 }
 
 .breadcrumbs a:hover {
-    background-color: #e3f2fd;
+    background-color: var(--verso-selected-color, #e3f2fd);
     text-decoration: underline;
 }
 
 .breadcrumbs .current {
     font-weight: 600;
-    color: #495057;
+    color: var(--verso-text-color, #495057);
     padding: 0.25rem 0.5rem;
     border-radius: 0.25rem;
 }
@@ -194,8 +195,8 @@ body {
     flex: 1;
     overflow-y: auto;
     padding: 2rem;
-    background: #ffffff;
-    color: #24292e;
+    background: var(--verso-background-color, #ffffff);
+    color: var(--verso-text-color, #24292e);
     font-family: "Monaco", "Menlo", "Ubuntu Mono", monospace;
     font-size: 1rem;
 }
@@ -206,7 +207,7 @@ body {
     margin-left: calc(var(--indent, 0) * 1ch);
     padding: 0.25rem 0.5rem;
     max-width: 40em;
-    border: 1px solid #ddd;
+    border: 1px solid var(--verso-border-color-light, #ddd);
     border-radius: 1rem;
     width: max-content;
 }
@@ -297,19 +298,19 @@ pre,
     top: 1rem;
     left: 1rem;
     z-index: 1001;
-    background: #fff;
-    border: 1px solid #dee2e6;
+    background: var(--verso-background-color, #fff);
+    border: 1px solid var(--verso-border-color, #dee2e6);
     border-radius: 0.375rem;
     padding: 0.5rem;
     cursor: pointer;
-    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
+    box-shadow: 0 2px 4px var(--verso-shadow-color, rgba(0, 0, 0, 0.1));
 }
 
 .hamburger span {
     display: block;
     width: 20px;
     height: 2px;
-    background: #333;
+    background: var(--verso-text-color, #333);
     margin: 4px 0;
     transition: 0.3s;
 }
@@ -327,7 +328,7 @@ pre,
         height: 100vh;
         z-index: 1000;
         transition: left 0.3s ease;
-        box-shadow: 2px 0 10px rgba(0, 0, 0, 0.1);
+        box-shadow: 2px 0 10px var(--verso-shadow-color, rgba(0, 0, 0, 0.1));
     }
 
     .menu-toggle:checked + .hamburger + .layout .sidebar {

src/verso-manual/VersoManual/Html/Style.lean

@@ -24,7 +24,7 @@ public def pageStyle : String := r####"
     --verso-logo-height: var(--verso-header-height);
 
     /** Table of Contents appearance **/
-    --verso-toc-background-color: #fafafa;
+    --verso-toc-background-color: var(--verso-surface-color, #fafafa);
     --verso-toc-text-color: var(--verso-text-color);
 
     /* How long should the ToC animation take? */
@@ -33,15 +33,15 @@ public def pageStyle : String := r####"
     /* How wide should the ToC be on non-mobile? */
     --verso-toc-width: 18rem;
 
-    /** Variables that control the “burger menu” appearance **/
+    /** Variables that control the "burger menu" appearance **/
     --verso-burger-height: 1.25rem;
     --verso-burger-width: 1.25rem;
     --verso-burger-line-width: 0.2rem;
     --verso-burger-line-radius: 0.2rem;
     --verso-burger-toc-visible-color: var(--verso-toc-text-color);
-    --verso-burger-toc-visible-shadow-color: #ffffff;
-    --verso-burger-toc-hidden-color: #0e2431;
-    --verso-burger-toc-hidden-shadow-color: #ffffff;
+    --verso-burger-toc-visible-shadow-color: var(--verso-background-color, #ffffff);
+    --verso-burger-toc-hidden-color: var(--verso-text-color, #0e2431);
+    --verso-burger-toc-hidden-shadow-color: var(--verso-background-color, #ffffff);
 
     /* The "burger menu" may need to get bigger for mobile screens */
     --verso-mobile-burger-height: 1.5rem;
@@ -85,6 +85,8 @@ html {
 body {
     margin: 0;
     padding: 0;
+    background-color: var(--verso-background-color, #ffffff);
+    color: var(--verso-text-color, #333333);
 }
 
 /******** Theme ********/
@@ -94,6 +96,7 @@ h1, h2, h3, h4, h5, h6 {
     font-weight: bold;
     text-rendering: optimizeLegibility;
     margin-top: 1.5rem;
+    color: var(--verso-structure-color, inherit);
 }
 
 p, dt, dd {
@@ -116,6 +119,14 @@ pre, code {
     overflow-y: clip;
 }
 
+a {
+    color: var(--verso-link-color, #386ee0);
+}
+
+a:hover {
+    color: var(--verso-link-hover-color, #0073e6);
+}
+
 /******** Page Layout ********/
 
 header {
@@ -124,11 +135,11 @@ header {
 	z-index: 99;
 	left: 0;
 	right: 0;
-	background: white;
+	background: var(--verso-background-color, white);
 	display: flex;
 	align-items: center;
 	height: var(--verso-header-height);
-	box-shadow: 0 0px 6px lightgray;
+	box-shadow: 0 0px 6px var(--verso-shadow-color, lightgray);
 }
 
 .header-logo-wrapper {
@@ -158,7 +169,7 @@ header {
 
 .header-title {
     text-decoration: none;
-    color: black;
+    color: var(--verso-text-color, black);
     font-size: 2rem;
     font-weight: bold;
     display: block;
@@ -240,7 +251,7 @@ main [id] {
     body:has(#toggle-toc:checked) .toc-backdrop {
         position: fixed;
         inset: 0;
-        background-color: #aaa8;
+        background-color: rgba(0, 0, 0, 0.5);
         z-index: 9;
     }
     html:has(#toggle-toc:checked) {
@@ -275,13 +286,13 @@ main [id] {
 }
 
 #toc a {
-    color: #333;
+    color: var(--verso-text-color, #333);
     text-decoration: none;
 }
 
 #toc a:hover {
     text-decoration: underline;
-    color: #000;
+    color: var(--verso-link-hover-color, #000);
 }
 
 .toc-title {
@@ -345,7 +356,7 @@ main [id] {
     width: calc(var(--verso-toc-triangle-width) + var(--verso-toc-triangle-left-space));
     height: var(--verso-toc-triangle-height);
     display: inline-block;
-    background-color: black;
+    background-color: var(--verso-text-color, black);
     content: ' ';
     transition: ease 0.2s;
     margin-right: var(--verso-toc-triangle-margin);
@@ -386,7 +397,7 @@ main [id] {
 }
 
 #toc .split-toc table {
-    border-left: 1px dotted;
+    border-left: 1px dotted var(--verso-border-color, currentColor);
     padding-left: 1.2rem;
     padding-top: 0.2rem;
 }
@@ -396,7 +407,7 @@ main [id] {
 }
 
 #toc .split-toc > ol {
-    border-left: 1px dotted;
+    border-left: 1px dotted var(--verso-border-color, currentColor);
     list-style-type: none;
     padding-left: 0.5rem;
     margin-left: 0;
@@ -443,7 +454,7 @@ main [id] {
     flex: 1;
     justify-content: center;
     align-items: center;
-    color: black;
+    color: var(--verso-text-color, black);
     text-decoration: none;
 }
 
@@ -798,7 +809,7 @@ main .section-toc a:hover {
 .permalink-widget > a {
   /* Don't show the colors here */
   color: transparent;
-  text-shadow: 0 0 0 gray;
+  text-shadow: 0 0 0 var(--verso-text-color-light, gray);
   text-decoration: none;
 }
 
@@ -881,18 +892,18 @@ main section li ol {
     overflow-x: auto;
     margin: 0px;
     margin: 0.5em .85em;
-    border-left: 0.2em solid red;
+    border-left: 0.2em solid var(--verso-error-indicator-color, red);
     padding: 0 0.45em;
 }
 
 /* Different color for warning */
 .lean-output.warning {
-    border-color: var(--verso-warning-color);
+    border-color: var(--verso-warning-indicator-color);
 }
 
 /* Different color for information */
 .lean-output.information {
-    border-color: #0000c0;
+    border-color: var(--verso-info-indicator-color, #0000c0);
 }