You will have to do it using CSS. First create the radio box field – I’ll use three options, Red, Green, and Blue – then inspect the post creation page and you will find something like this:
<div id="acf-radio" class="field field_type-radio field_key-field_53afc2e795462" data-field_name="radio" data-field_key="field_53afc2e795462" data-field_type="radio">
<p class="label"><label for="acf-field-radio">Radio</label></p>
<ul class="acf-radio-list radio horizontal">
<li><label><input id="acf-field-radio-Red" type="radio" name="fields[field_53afc2e795462]" value="Red" checked=""checked"" data-checked=""checked"">Red</label></li>
<li><label><input id="acf-field-radio-Blue" type="radio" name="fields[field_53afc2e795462]" value="Blue">Blue</label></li>
<li><label><input id="acf-field-radio-Green" type="radio" name="fields[field_53afc2e795462]" value="Green">Green</label></li>
</ul>
</div>
As you can see, each input has a unique ID which can be targeted with CSS styling. You might try a technique such as this one on TutsPlus.
