/* убираем стандартную стрелку */
.t-menusub__arrow {
    background: none !important;
    transform: none !important;
    position: relative;
}

/* сам плюс */
.t-menusub__arrow:before {
    content: "+";
    display: block;
    font-size: 30px;
    line-height: 24px;
    font-family: Arial, sans-serif;
    transition: transform 0.25s ease;
    transform: rotate(0deg);
}

/* убираем лишнее */
.t-menusub__arrow:after {
    content: none !important;
}

/* =============================== */
/* ВАЖНО: поворот через родителя   */
/* =============================== */

.t-menusub__list-item_open .t-menusub__arrow:before {
    transform: rotate(45deg);
}