Deprecate bunch of styles

All of these have built-in support nowadays

chrome/deprecated/blank_page_background.css

@@ -0,0 +1,9 @@
+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/blank_page_background.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+/* Set blank page background-color */
+/* Uses color from theme_colors if available */
+
+#tabbrowser-tabpanels{
+  background-color: var(--uc-light-bkgnd-color,rgb(46,54,69)) !important;
+}