styles: Convert [readonly] selector to :read-only pseudo-class.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/styles/night_mode.css

@@ -116,7 +116,8 @@ on a dark background, and don't change the dark labels dark either. */
 
     /* these are converting grey things to "new grey" */
     :disabled,
-    *[readonly="readonly"],
+    input:read-only,
+    textarea:read-only,
     .sidebar-title,
     .recipient_row_date {
         color: inherit;