I’m no css wizard but this would be my solution… Just change the colours to whatever you want to match your theme colours better from below. Also I’d recommend reading this topic its was very helpful to me in the past as it helped me learn to do cool stuff with my site.
Bronze
.badge-card .badge-contents .badge-icon.badge-type-bronze .fa {
color: #cd7f32 !important;
}
Silver
.badge-card .badge-contents .badge-icon.badge-type-silver .fa {
color: silver !important;
}
Gold
.badge-card .badge-contents .badge-icon.badge-type-gold .fa {
color: #e7c300 !important;
}