How can I add custom css styles to radio buttons?

In two steps:

1. The first step is adding the Container CSS Class: custom-control to the radio button field

– https://docs.easyforms.dev/form-builder.html#custom-radio-button

2. Then, you need use a CSS Theme to inject the CSS styles to your form (color, size, etc.). For example, to display a radio button in red color you can use:

.custom-control .radio input:checked+label::before,
.custom-control .checkbox input:checked+label::before,
.custom-control .inline-control-radio input:checked+label::before,
.custom-control .inline-control-checkbox input:checked+label::before
    border-color: red;
}

input:checked+label::after, 
.custom-control .inline-control-radio input:checked+label::after, 
.custom-control .inline-control-checkbox input:checked+label::after {
    content: '';
    display: inline-block;
    position: absolute;
    width: 6px;
    height: 6px;
    left: 4px;
    top: 8px;
    margin-left: -16px;
    border: 1px solid red;
    border-radius: 50%;
    background-color: red;
}

Related documentation |  CSS Themes Management