3.4 #Forms.Option checkbox Option checkbox
Add .cs inside .cs-checkbox to <input type="checkbox"> elements to apply the styles.
Example
Markup
<div class="cs-checkbox"><label><input type="checkbox" class="cs"><span class="lbl">Check this</span></label></div><div class="cs-checkbox"><label><input type="checkbox" class="cs" checked><span class="lbl">Or check that</span></label></div><div class="cs-checkbox"><label><input type="checkbox" class="cs" disabled><span class="lbl">This option is disabled</span></label></div>
Source:
3_Form/forms.less, line 60