.label--checkbox input[type="checkbox"]{...}
.b-sidebar__menu__item::after{
position: absolute;
top: 24px;
content: "";
width: 8px;
height: 8px;
border: 1px solid #000000;
border-width: 1px 0 0 1px;
transform: rotate(135deg);
right: 20px;
}
.b-sidebar__menu__item--clicked::after{
transform: rotate(77deg);
}