How to customize elements in a #shadow-root (open)

I am trying to customize a third-party code to match the colors on our Squarespace website. The third party has an open #shadow-root on their Footer, but I’m still unable to customize part of the code. What I’m trying to do is change the background-color to #ff0000;

I’ve tried to target .formFooter in CSS, but from what I’ve tried, it hasn’t worked. Here is their code below through Inspector:

<div>
 #shadow-root (open)
 <style></style>
<div class="formFooter-heightMa sk"></div>
<div class="formFooter f6 branding21 ">∞</div>flex)
== $0
<style id="action-styles">
</style>
</div>
</form>

And here is their style:

.formFooter {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
    background-color: #0A1551;