Or you can just do inline styles defined inside the anchor tag. Code will be more messy but something like this should work:

Not a member yet? <a href="{0}">
<span style="
background-color: #4c679f;
color: white;
font-size: 1rem;
font-weight: 700;
border-radius: 3px;
padding: 6px 8px;
text-decoration: none;
text-transform: uppercase;">Sign up</span>
</a>