You can put an Font Awsome Icon insede the <a> tag, like this:

<a data-toggle="collapse" href="#ID" role="button" aria-expanded="false" aria-controls="ID">
     <i class="fal fa-sun"></i>    
</a>

You can format the icon any way you want, by default it will be the standard link color

Exemple:

<a data-toggle="collapse" href="#ID" role="button" aria-expanded="false" aria-controls="ID">
   <i class="fal fa-triangle fa-flip-vertical text-danger"></i>    
</a>

<div class=" collapse " id="ID">
   <p>Hello!</p> 
</div>

Hope it helps! I'll also put an explanation on the sheet