style: add dark style for `pre` and `code` (#6382)

site/src/theme/static/highlight.dark.less

@@ -0,0 +1,16 @@
+html[data-doc-theme='dark'] {
+  pre code {
+    background: #262626;
+    border-color: #262626;
+  }
+
+  code[class*='language-'],
+  pre[class*='language-'] {
+    color: black;
+  }
+
+  :not(pre) > code[class*='language-'],
+  pre[class*='language-'] {
+    background: #262626;
+  }
+}

site/src/theme/static/index.less

@@ -9,6 +9,7 @@
 @import './toc';
 @import './not-found';
 @import './highlight';
+@import './highlight.dark';
 @import './demo';
 @import './icons';
 @import './mock-browser';