Section 5.6 Text Weight
Adjust the font-weight:
100
200
300
400
500
600
700
800
900
<div style="font-family: 'Avenir Next', 'Helvetica Neue', sans-serif">
<p class="fw1">100</p>
<p class="fw2">200</p>
<p class="fw3">300</p>
<p class="fw4">400</p>
<p class="fw5">500</p>
<p class="fw6">600</p>
<p class="fw7">700</p>
<p class="fw8">800</p>
<p class="fw9">900</p>
</div>text.css, line 222