I struggled for hours trying to figure out how to customize Bulma. Here's a quick solution.
You need to verify that the variable you are trying to change exists in the component you are including. For example, click on it in VSC and inspect its code if needed.
To change the hover color of the breadcrumb component:
@use "bulma/sass/components" with (
$breadcrumb-item-hover-color: #1447e6
);