It's pretty easy to do. Just add the below bit of CSS code to body.html:
<style>
/* add labels to toolbar */
.toolbox-button {
width: 7vw;}
.toolbox-button:after {
content: attr(aria-label);
font-size: 0.7rem;
line-height: 0.8rem;
}
.toolbox-icon {
width: 100%; }
.welcome-page-settings .toolbox-button {
display: flex;}
.reactions-row .toolbox-button { width: 30%; }
.reactions-row .toolbox-icon span.emoji { width: 5rem; }
.reactions-row .toolbox-icon { width: 5rem; }
</style>