Another thing to note, there is an extra class composing-whisper applied when you toggle it on, so you could make the preview more obvious too if you want.
Currently it does apply font-style: italic; to the preview but you could also add something to make it even more super obvious like:
.composing-whisper .d-editor-preview {
background: #960000;
padding: 20px;
border: 5px solid #ffd600;
}
I guess this wouldn’t be useful for replies on mobile. But yeah changing the button I think would be an easy CSS tweak as well!