Thanks. In /components/com_easydiscuss/themes/simplistic I renamed the original as "subscription-orig.php" and uploaded your new one. Its slightly larger and has today's date. But nothing seemed to have changed. The subscribe to a thread is still a little envelope.
btw, I cleared Joomla's cache as well as my browser's cache.
As a side note, the example page I cited,
https://samraksh.com/support/forum/report-bug-data-collection-app-note-for-radar, has 3 envelope icons on it: one to subscribe to the site, one to view conversations, and one to subscribe to the thread. Each has a different meaning and it's hard to understand what each one means.