core.css

* {
    box-sizing: border-box;
}
body {
    margin: 0;

    background: #f1f1f1;
    font-family: sans-serif;

    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
}

a {
    text-decoration: none;
}

header {
    display: flex;
    justify-content: flex-start;

    width: 100%;
    height: 50px;
    position: fixed;
    z-index: 200;
    background: #444;
}

header a {
    display: block;
    position: relative;

    top: 15px;
    right: 10px;
    width: 25px;
    height: 40px;
}

main {
    width: 100%;
    padding: 30px;
    padding-top: 80px;
    margin-left: 250px;
    transition: margin-left .5s;
}

aside {
    width: 250px;
    height: 100%;
    margin-top: 50px;
    left: 0;
    position: fixed;
    background: #f3f3f3;
    transition: position, left .5s;
}

.main-toggle {
    margin-left: 0;
    transition: margin-left .5s;
}
.aside-toggle {
    left: -250px;
    transition: position, left .5s;
}

.foto-perfil {
    width: 100%;
    height: 200px;
    position: relative;
    border-bottom: 1px solid #aaa;
}

.imagen {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto;

    width: 150px;
    height: 150px;
    border-radius: 50%;

    box-shadow: 0 0 3px 3px rgba(0,0,0,.5   );
}

.menu-aside a {
    display: block;
    padding: 10px;
    border-bottom: 1px solid #aaa;
    color: #444;
}
.menu-aside a:hover {
    background: rgba(0,0,0,.1);
}
@media only screen and (max-width: 900px) {
  aside{
        position: fixed;
        left:-250px;
        transition:position, left 0.5s;
    }
    .aside-toggle{
        position: fixed;
        left:0px;
        transition:position, left 0.5s;
    }
    main{
        margin-left:0px;
        transition:margin-left 0.5s;
    }
}