Add a css class to a OOTB component as a parameter -- or anything, really?

Create a component called ootb.widget@override (or whatever your component is)

inside the contents you would put

<div class="custom-css-class">
<@delegate />
</div>

then it would render all the css classes under the custom-css-class. 

so.. 

<span class="lia-thread-solved lia-component-solution-link">
<a class="lia-link-navigation lia-forum-topic-page-solution-link" id="link_7" href="#somelink">Go to Solution</a>
</span>

would then be

<div class="custom-css-class">
<span class="lia-thread-solved lia-component-solution-link">
<a class="lia-link-navigation lia-forum-topic-page-solution-link" id="link_7" href="#somelink">Go to Solution</a>
</span>
</div>

 at which point you could just use css...

.custom-css-class .lia-thread-solved {

font-size:100px;

}