Use default color and background in third-party github-markdown.css (#8515)

Author: isoos

third_party/css/github-markdown.css

@@ -66,6 +66,11 @@
   padding: 0 1em;
   color: #6a737d;
   border-left: 0.25em solid #dfe2e5;
+
+  /* Note: pub.dev override. */
+  .dark-theme & {
+    color: inherit;
+  }
 }
 
 .markdown-body blockquote>:first-child {
@@ -88,6 +93,13 @@
   border-bottom-color: #959da5;
   border-radius: 3px;
   box-shadow: inset 0 -1px 0 #959da5;
+
+  /* Note: pub.dev override. */
+  .dark-theme & {
+    color: inherit;
+    background-color: inherit;
+    box-shadow: none;
+  }
 }
 
 .markdown-body h1,
@@ -144,6 +156,11 @@
 .markdown-body h6 {
   font-size: 0.85em;
   color: #6a737d;
+
+  /* Note: pub.dev override. */
+  .dark-theme & {
+    color: inherit;
+  }
 }
 
 .markdown-body ul,
@@ -261,6 +278,11 @@
   padding: 5px 0 0;
   clear: both;
   color: #24292e;
+
+  /* Note: pub.dev override. */
+  .dark-theme & {
+    color: inherit;
+  }
 }
 
 .markdown-body span.align-center {