.box {
    width: 600px;
    height: 600px;
    outline: 1px solid green;
    position: relative;
}

.inner-box {
    width: 50px;
    height: 50px;
    outline: 1px solid red;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-top: ;
    margin-left: -50%;
}

.portfolio-list {
    display: flex;
    flex-wrap: wrap;
}

.list-item {
    flex-basis: calc((100% - 60px) / 3);
    border-bottom: 1px solid var(--border-color);
}

.list-item:not(:nth-child(3n)) {
    margin-right: var(--card-gap);
}

.list-item:not(:nth-last-child(-n + 3)) {
    margin-bottom: var(--card-gap);
}

.ft-contact-item + .ft-contact-item {
    margin-top: 9px;
}

img {
    display: block;
    max-width: 100%;
    height: auto;
}

.header-contact.mail::before {
    display: inline-flex;
    flex-direction: row;
    content: '';
    background-image: url(../images/mail.svg);
    width: 16px;
    height: 12px;
    background-repeat: no-repeat;
    background-size: contain;
    margin-right: 10px;
}

.header-contact:hover .mail-icon {
    fill: var(--accent-color);
}