That alone seems to get overridden:

But adding !important to white-space does work:
.horizontal-menu-menu-item span{
white-space: pre-wrap !important;
word-break: break-word;
margin:auto;
justify-content: center;
}
Thanks again, you’re helping me out all over the place 
Edit: Adding justify-content to make sure the content on all buttons is centered. Without it, just the text on buttons with multiple lines gets centered.