Thanks Dan for reaching out. I got help from Zendesk support with this tip and that works.

What I ended up is to include the actual HTML in my style.css, like this:

.OrangeButton {
color: white;
background-color: #d86633;
cursor: pointer;
border-radius: 32px;
border: 1px solid #d86633;
padding: 8px 16px;
}
.OrangeButton:hover {color: #d86633;background-color: white;}

... and then this in my header.hbs:

<a class="OrangeButton" href="https://www.ndcsolutions.com/partner-portal ">Visit the Partner Portal</a>

...so now I am a css guru!