To not edit simplecss, add a custom.css instead

custom.css

@@ -0,0 +1,8 @@
+:root {
+  --code: #212121;
+}
+@media (prefers-color-scheme: dark) {
+  :root {
+    --code: #212121;
+  }
+}

publish.el

@@ -44,8 +44,11 @@
 	 :html-head "<link rel=\"stylesheet\"
 	     href=\"simple.min.css\"
 	     type=\"text/css\"/>
-	     <link rel=\"stylesheet\"
+<link rel=\"stylesheet\"
 	     href=\"htmlize.min.css\"
+	     type=\"text/css\"/>
+<link rel=\"stylesheet\"
+	     href=\"custom.css\"
 	     type=\"text/css\"/>")))
 
 (org-publish "tlakh")