firefox: add user CSS files

firefox/userChrome.css

+/*
+ * https://support.mozilla.org/en-US/questions/1269392
+ * http://kb.mozillazine.org/UserContent.css
+ *
+ * about:config -> toolkit.legacyUserProfileCustomizations.stylesheets = true
+ */
+@-moz-document url(chrome://browser/content/browser.xhtml) {
+    #main-window,
+    browser[type="content-primary"],
+    browser[type="content"],
+    tabbrowser#content,
+    #content,
+    browser[type="content"] > html {
+        background: #323234 !important;
+    }
+}

firefox/userContent.css

+@charset "utf-8";
+
+@-moz-document url("about:newtab") {
+    body { background-color: #011326 !important;}
+}
+
+@-moz-document url(chrome://browser/content/browser.xhtml) {
+    browser[type="content-primary"] {background: #323234 !important}
+}