Bug 1653751 - Inject a rule into junk log to style it with dark system theme. r=mkmelin DONTBUILD

mailnews/base/content/junkLog.js

@@ -12,12 +12,16 @@ function onLoad() {
   gLogView = document.getElementById("logView");
   gLogView.docShell.allowJavascript = false; // for security, disable JS
 
+  gLogView.addEventListener("load", () => {
+    addStyling();
+  });
+
   gLogFile = Services.dirsvc.get("ProfD", Ci.nsIFile);
   gLogFile.append("junklog.html");
-
   if (gLogFile.exists()) {
-    // convert the file to a URL so we can load it.
     gLogView.setAttribute("src", Services.io.newFileURI(gLogFile).spec);
+  } else {
+    addStyling(); // set style for initial about:blank
   }
 }
 
@@ -27,3 +31,14 @@ function clearLog() {
     gLogView.setAttribute("src", "about:blank"); // we don't have a log file to show
   }
 }
+
+function addStyling() {
+  let style = gLogView.contentDocument.createElement("style");
+  gLogView.contentDocument.head.appendChild(style);
+  style.sheet.insertRule(
+    `@media (prefers-color-scheme: dark) {
+       :root { scrollbar-color: rgba(249, 249, 250, .4) rgba(20, 20, 25, .3);}
+       body { color: #f9f9fa;}
+     }`
+  );
+}