.checked {
    ...
    background-position: 12px 2px;
}

.checked {
    background: url(data:image/svg+xml;utf8;base64,PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iaXNv…0wLjEtMC43LTAuM2wtNy44LTguNC0uMi0uM3oiIGZpbGw9IiMwMDAwMDAiLz4KPC9zdmc+Cg==) 12px 2px no-repeat / 20px;
    ...
}

<label class="checkbox-label">
    <input type="checkbox"><i></i>
    Пицца
</label>

.checkbox-label input {
    opacity: 0;
    position: absolute;
}
.checkbox-label i {
    display: inline-block;
    width: 20px;
    height: 20px;
    border: 2px solid black;
    vertical-align: middle;
    margin-right: 5px;
    background-color: #fff;
}
.checkbox-label input:checked + i {
    background: url(data:image/svg+xml;utf8;base64,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) no-repeat center center #fff;
    background-size: 15px;
}