inventory.css

.mcui-inventory .mcui-inventory-row {
    display: block;
}

.mcui-inventory-hotbar {
    padding-top: 8px;
}

.mcui-inventory-slot {
    position: relative;
    display: inline-block;
    background: #8b8b8b no-repeat center center / 32px 32px;
    border: 2px solid;
    border-color: #373737 #fff #fff #373737;
    width: 32px;
    height: 32px;
    font-size: 16px;
    line-height: 1;
    text-align: left;
    vertical-align: bottom;
}

.mcui-inventory-slot::before, .mcui-inventory-slot::after {
    content: "";
    position: absolute;
    background-color: #8b8b8b;
    height: 2px;
    width: 2px;
    pointer-events: none;
}

.mcui-inventory-slot::before {
    bottom: -2px;
    left: -2px;
}

.mcui-inventory-slot::after {
    top: -2px;
    right: -2px;
}

.mcui-inventory-slot-large {
    padding: 8px;
}

.mcui-inventory-item {
    position: relative;
    display: block;
    margin: -2px;
    padding: 2px;
    width: 32px;
    height: 32px;
}

.mcui-inventory-stacksize {
    position: absolute;
    right: 0;
    bottom: 0;
    font-family: Minecraftia, sans-serif !important;
    font-weight: normal !important;
    color: #fff !important;
    text-shadow: 2px 2px 0 #3f3f3f;
    z-index: 2;
}