Bootstrap-4 carousel is ignoring my custom css

Below are the carousel arrows which are part of a larger carousel object which I'm trying to customise through my external custom.css:

<!-- Left and right controls -->
<a class="carousel-control-prev" href="#demo" data-slide="prev">
<span class="carousel-control-prev-icon"></span>
</a>
<a class="carousel-control-next" href="#demo" data-slide="next">
<span class="carousel-control-next-icon"></span>
</a>

Code (CSS):

I've, in fact, even tried overqualifying the .carousel-control-next-icon...

Bootstrap-4 carousel is ignoring my custom css

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