Hello,
All other parts of the design can be changed using the same way, you just need to the check css classes via browsers console and then to overwrite existed class in studio->design->injections.

Example how to replace send button:

.bx-messenger-post-box-send-button .sys-icon.arrow-up::before,
.bx-messenger-post-box-send-button .fa-arrow-up::before {
content: url(../../modules/boonex/messenger/template/images/icons/send.png);
}