wagtail/client/scss/components/_button-select.scss

.button-select {
    &__option {
        display: block;
        width: 100%;
        margin-bottom: 10px;

        background-color: #fff;
        border-color: $color-teal;
        color: #262626;

        &--selected {
            background-color: $color-teal;
            color: #fff;
        }
    }
}

.button-select .button-select__option {
    /* override default margin from horizontally-aligned buttons */
    margin-left: 0;
}