“posts_nav_link ()” in WordPress is not getting CSS

Good afternoon,
I am making a website in WordPress but I have the following problem: I want the navigation links between pages are not underlined (text-decoration: none), but I’ve tried several ways and I’m not getting. Can anyone help me?

My HTML:

<div class="page_navi">
  <?php posts_nav_link('<span class="span_navi">sep</span>','<span class="span_navi">Página Anterior</span>','<span class="span_navi">Proxima Página</span>') ?></div>

My CSS:

    .page_navi {
    font-family: CreativZoo Regular, sans-serif;
    float: right;
    margin-right: 20px;
    margin-bottom: 10px;
    font-size: 20px;
    color: #136f8c;
    text-decoration: none;
    }

    .page_navi span {
    text-decoration: none;
    }

    .span_navi {
    font-size: 20px;
    color: #fff;
    text-decoration: none;
    }

    .span_navi:hover {
    font-size: 20px;
    color: #136f8c;
    text-decoration: none;
    }