Css change

[Dawid Glomba]

where change border-radius: ;

[yariplus]

You could do something like this.

[component="categories/category"] .fa-stack {
    margin-right: 5px;

    .fa-circle::before {
        content: "\0000a0";
        width: 24px;
        background-color: rebeccapurple;
        display: block;
        border-radius: 5px;
    }
}

It's kindof a hack, you would need to modify the theme to get real control. Also, there's no way to get the category background.