You can provide formatting for Label content wherever you invoke text-link <$_prev> and <$_next> macros. For example:
<p class="navlabel"><$_prev></p>
Button labels are at the mercy of default browser rendering, unless you redefine <$_prev> and <$_next> macros so they include formatting. For buttons, formatting tags must be placed within the <button> tag, surrounding Label text. For example (must be all on one line):
<button type="button" onclick="javascript:location.href='somefile.htm'"> <p class="navlabel">Previous</p></button>