Update dark theme css to lower brightness to all icons (#5439)

* Update dark.css to lower brightness to all icons

* Update dark.rtl.css to lower brightness to all icons

* re-add p.help .icon

* re-add p.help .icon

p/themes/Dark/dark.css

@@ -110,7 +110,8 @@ img.favicon {
 
 body img,
 body video,
-p.help .icon {
+p.help .icon,
+.icon {
 	filter: brightness(.6) contrast(1.2);
 }
 

p/themes/Dark/dark.rtl.css

@@ -110,7 +110,8 @@ img.favicon {
 
 body img,
 body video,
-p.help .icon {
+p.help .icon,
+.icon {
 	filter: brightness(.6) contrast(1.2);
 }