css: Introduce dark mode

This patch extends our CSS to introduce dark mode, so the style shown
matches the user media preference.

It is very analogous to the previous one, only minor adjustments have
been made to make the contrast levels pass the accessibility standards.

No changes have been made to the pygments CSS. It works surprisingly
well as-is, but there are some minor changes that may be needed. Those
will be done in subsequent patches.

static/git-arr.css

@@ -1,13 +1,53 @@
 /*
  * git-arr style sheet
  */
+:root {
+    --body-bg: white;
+    --text-fg: black;
+    --h1-bg: #ddd;
+    --hr-bg: #e3e3e3;
+    --text-lowcontrast-fg: grey;
+    --a-fg: #800;
+    --a-explicit-fg: #038;
+    --table-hover-bg: #eee;
+    --head-bg: #88ff88;
+    --tag-bg: #ffff88;
+    --age-fg0: darkgreen;
+    --age-fg1: green;
+    --age-fg2: seagreen;
+    --diff-added-fg: green;
+    --diff-deleted-fg: red;
+}
+
+@media (prefers-color-scheme: dark) {
+    :root {
+        --body-bg: #121212;
+        --text-fg: #c9d1d9;
+        --h1-bg: #2f2f2f;
+        --hr-bg: #e3e3e3;
+        --text-lowcontrast-fg: grey;
+        --a-fg: #d4b263;
+        --a-explicit-fg: #44b4ec;
+        --table-hover-bg: #313131;
+        --head-bg: #020;
+        --tag-bg: #333000;
+        --age-fg0: #51a552;
+        --age-fg1: #468646;
+        --age-fg2: #2f722f;
+        --diff-added-fg: #00A000;
+        --diff-deleted-fg: #A00000;
+    }
+}
+
 body {
     font-family: sans-serif;
     padding: 0 1em 1em 1em;
+    color: var(--text-fg);
+    background: var(--body-bg);
 }
 
 h1 {
-    background: #ddd;
+    background: var(--h1-bg);
     padding: 0.3em;
 }
 
@@ -19,7 +59,7 @@ h2, h3 {
 
 hr {
     border: none;
-    background-color: #e3e3e3;
+    background-color: var(--hr-bg);
     height: 1px;
 }
 
@@ -27,22 +67,21 @@ hr {
 /* By default, use implied links, more discrete for increased readability. */
 a {
     text-decoration: none;
-    color: black;
+    color: var(--text-fg);
 }
 
 a:hover {
-    text-decoration: underline;
-    color: #800;
+    color: var(--a-fg);
 }
 
 
 /* Explicit links */
 a.explicit {
-    color: #038;
+    color: var(--a-explicit-fg);
 }
 
 a.explicit:hover, a.explicit:active {
-    color: #880000;
+    color: var(--a-fg);
 }
 
 
@@ -63,14 +102,14 @@ table.nice td.main {
 }
 
 table.nice tr:hover {
-    background: #eee;
+    background: var(--table-hover-bg);
 }
 
 
 /* Table for commits. */
 table.commits td.date {
     font-style: italic;
-    color: gray;
+    color: var(--text-lowcontrast-fg);
 }
 
 @media (min-width: 600px) {
@@ -80,7 +119,7 @@ table.commits td.date {
 }
 
 table.commits td.author {
-    color: gray;
+    color: var(--text-lowcontrast-fg);
 }
 
 
@@ -94,7 +133,7 @@ table.commit-info td {
 }
 
 table.commit-info span.date, span.email {
-    color: gray;
+    color: var(--text-lowcontrast-fg);
 }
 
 
@@ -102,21 +141,21 @@ table.commit-info span.date, span.email {
 span.refs {
     margin: 0px 0.5em;
     padding: 0px 0.25em;
-    border: solid 1px gray;
+    border: solid 1px var(--text-lowcontrast-fg);
 }
 
 span.head {
-    background-color: #88ff88;
+    background-color: var(--head-bg);
 }
 
 span.tag {
-    background-color: #ffff88;
+    background-color: var(--tag-bg);
 }
 
 
 /* Projects table */
 table.projects td.name a {
-    color: #038;
+    color: var(--a-explicit-fg);
 }
 
 
@@ -124,20 +163,20 @@ table.projects td.name a {
  * Note this is hidden by default as we rely on javascript to show it. */
 span.age {
     display: none;
-    color: gray;
+    color: var(--text-lowcontrast-fg);
     font-size: smaller;
 }
 
 span.age-band0 {
-    color: darkgreen;
+    color: var(--age-fg0);
 }
 
 span.age-band1 {
-    color: green;
+    color: var(--age-fg1);
 }
 
 span.age-band2 {
-    color: seagreen;
+    color: var(--age-fg2);
 }
 
 
@@ -171,11 +210,11 @@ table.changed-files {
 }
 
 table.changed-files span.lines-added {
-    color: green;
+    color: var(--diff-added-fg);
 }
 
 table.changed-files span.lines-deleted {
-    color: red;
+    color: var(--diff-deleted-fg);
 }
 
 
@@ -185,7 +224,7 @@ div.paginate {
 }
 
 div.paginate span.inactive {
-    color: gray;
+    color: var(--text-lowcontrast-fg);
 }
 
 
@@ -202,7 +241,7 @@ table.ls {
 }
 
 table.ls tr.blob td.size {
-    color: gray;
+    color: var(--text-lowcontrast-fg);
 }
 
 
@@ -219,8 +258,8 @@ table.blob-binary pre {
 table.blob-binary .offset {
     text-align: right;
     font-size: x-small;
-    color: darkgray;
-    border-right: 1px solid #eee;
+    color: var(--text-lowcontrast-fg);
+    border-right: 1px solid var(--text-lowcontrast-fg);
 }
 
 table.blob-binary tr.etc {
@@ -236,7 +275,7 @@ div.linenodiv {
 }
 
 div.linenodiv a {
-    color: gray;
+    color: var(--text-lowcontrast-fg);
 }
 
 div.source_code {
@@ -262,7 +301,7 @@ table.repo_info td {
 }
 
 span.ctrlchr {
-    color: gray;
+    color: var(--text-lowcontrast-fg);
     padding: 0 0.2ex 0 0.1ex;
     margin: 0 0.2ex 0 0.1ex;
 }
@@ -274,11 +313,11 @@ span.ctrlchr {
 
 /* Colored links (same as explicit links above) */
 div.markdown a {
-    color: #038;
+    color: var(--a-explicit-fg);
 }
 
 div.markdown a:hover, div.markdown a:active {
-    color: #880000;
+    color: var(--a-fg);
 }