* {
    margin: 0;
    padding: 0;
    user-select: none;
    box-sizing: border-box;
    font-family: Arial, Helvetica, sans-serif;
}

body {
    overflow: hidden;
}

.btn {
    position: absolute;
    top: 15px;
    left: 15px;
    height: 45px;
    width: 45px;
    text-align: center;
    background: #1b1b1b;
    border-radius: 3px;
    cursor: pointer;
    transition: left 0.4s ease;
}

.btn.click {
    left: 425px;
}

.btn span {
    color: white;
    font-size: 28px;
    line-height: 45px;
}

.btn.click span:before {
    content: '\f00d'
}

.sidebar.show {
    left: 0px;
}

.sidebar {
    position: fixed;
    width: 400px;
    height: 100%;
    overflow-y: auto;
    left: -400px;
    background: #1b1b1b;
    transition: left 0.4s ease;
}

.sidebar .text {
    color:white;
    font-size: 25px;
    font-weight: 600;
    line-height: 65px;
    text-align: center;
    background: #1e1e1e;
    letter-spacing: 1px;
}

.sidebar .text.smaller {
    font-size: 20px;
    font-weight: 300;
    line-height: 50px;
    padding-left: 80px;
    background: #1b1b1b;
    text-align: left;
}

.sidebar .text.choice {
    font-size: 20px;
    font-weight: 400;
    line-height: 50px;
    padding-left: 80px;
    background: #1b1b1b;
    text-align: left;
}

nav ul {
    background: #1b1b1b;
    height: 100%;
    width: 100%;
    list-style: none;
}

nav ul li {
    line-height: 60px;
    border-bottom: 1px solid rgba(255, 255, 255, 0.1);
}

nav ul li:last-child {
    border-bottom: 1px solid rgba(255, 255, 255, 0.05);
}

nav ul li a {
    position: relative;
    color: white;
    text-decoration: none;
    font-size: 18px;
    padding-left: 40px;
    font-weight: 500;
    display: block;
    width: 100%;
    border-left: 3px solid transparent;
}

nav ul li.active a, nav ul li.active button{
    color: cyan;
    background: #1b1b1b;
    border-left-color: cyan;
}

nav ul ul {
    position: static;
    display: none;
}


/* Checkbox Buttons */
.switch {
    position: relative;
    top: 9px;
    display: inline-block;
    width: 60px;
    height: 34px;
}

.switch input { 
    opacity: 0;
    width: 0;
    height: 0;
}

.slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #ccc;
    -webkit-transition: .4s;
    transition: .4s;
}

.slider:before {
    position: absolute;
    content: "";
    height: 26px;
    width: 26px;
    left: 4px;
    bottom: 4px;
    background-color: white;
    -webkit-transition: .4s;
    transition: .4s;
}

input:checked + .slider {
    background-color: #2196F3;
}

input:focus + .slider {
    box-shadow: 0 0 1px #2196F3;
}

input:checked + .slider:before {
    -webkit-transform: translateX(26px);
    -ms-transform: translateX(26px);
    transform: translateX(26px);
}

.slider {
    border-radius: 34px;
}

.slider:before {
    border-radius: 50%;
}

.overlay {
    position: fixed;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    background: rgba(0, 0, 0, 0.7);
    transition: opacity 500ms;
    visibility: hidden;
    opacity: 0;
}


/* Drop Down */

nav ul .feat-show.show {
    display: block;
}

nav ul .learn-show.show4 {
    display: block;
}

nav ul .def-show.show5 {
    display: block;
}

nav ul .formula-show.show7 {
    display: block;
}

nav ul .blackholedef-show.show8 {
    display: block;
}

nav ul .blackholehorizon-show.show9 {
    display: block;
}

nav ul .blackholesingularity-show.show10 {
    display: block;
}

nav ul .opt-show.show2 {
    display: block;
}

nav ul ul li {
    line-height: 42px;
    border-bottom: none;
}

nav ul ul li a, nav ul ul li button {
    cursor: pointer;
    font-size: 17px;
    color: #e6e6e6;
    padding-left: 80px;
}

nav ul li ul li a, nav ul li ul li button {
    color: #e6e6e6;
    border: none;
    background: #1b1b1b;
}

nav ul li.active ul li a, nav ul li.active ul li button {
    color: #e6e6e6;
    background: #1b1b1b;
    border-left-color: transparent;
}

nav ul ul li a:hover, nav ul ul li button:hover {
    color: cyan!important;
    background: #1b1b1b!important;
}

nav ul li a span, nav ul li button span {
    position: absolute;
    top: 50%;
    right: 20px;
    transform: translateY(-50%);
    font-size: 22px;
    transition: transform 0.4s;
}

nav ul li a span.rotate {
    transform: translateY(-50%) rotate(-180deg);
}