Hi tavasjn24,
It sounds like you have two options.
First, if you just wanted to wrap the custom content in a CSS class then you can add the wrapper attribute to the component in your quilt, as shown below.
<component id="custom_content_1" wrapper="my-spiffy-class"/>
Alternatively, if you wanted to call your custom content within a custom component and then just put that component in your quilt instead, then you could create a component that looks like the example below. (This example assumes you're wanting to render Custom Content #3.)
<div class="my-spiffy-custom-component">
<div class="my-spiffy-content">
<@component id="custom_content_3"/>
</div>
</div>
Hope this helps.