* { -webkit-box-sizing:border-box; -moz-box-sizing:border-box; -ms-box-sizing:border-box; -o-box-sizing:border-box; box-sizing:border-box; }

html { width: 100%; height:100%; overflow:auto; }
<link href="https://fonts.googleapis.com/css?family=Roboto:100,300,400,500,700,900" rel="stylesheet">

body {
	height: auto;
	font-family: 'Roboto', sans-serif;
    background-color: #FFFFFF;
}

/* --------------------------------------------------------- TEXT --------------------------------------------------- */
.text-center {
    text-align: center;
}
.text-left {
    text-align: left;
}
.text-right {
    text-align: right;
}

/* -------------------------------------------------------- HEADINGS ------------------------------------------------ */
.heading-medium {
    font-size: 2rem;
    font-weight: 500;
}
.heading-small {
    font-size: 1.5rem;
    font-weight: 400;
}
.heading-large {
    font-size: 2.5rem;
    font-weight: 700;
}
.heading-extra-large {
    font-size: 3.5rem;
    font-weight: 900;
}

/* --------------------------------------------------------- BUTTONS ------------------------------------------------ */
.btn-1 {
    background: #9f2ca2;
    color: white;
    border: none;
    transition: all 0.3s ease-in-out;
    padding:1% 0%;
    width: 100%;
    text-decoration: none;
}
.btn-1:hover {
    background: #77067a;
    transform: scale(1.02);
}

.btn-gray {
    border: none;
    background: #898989;
    color: white;
    width: 100%;
    padding: 1% 0%;
    text-decoration: none;
    transition: all 0.3s ease-in-out;
}
.btn-gray:hover {
    transform: scale(1.02);
}

/* --------------------------------------------------- FORMS-CONTAINERS --------------------------------------------- */
.full-page-form-container {
    display: flex;
    flex-direction: column;
    height: 90vh;
    justify-content: center;
    align-items: center;
}
.full-page-form-container form {
    display: flex;
    flex-direction: column;
    width: 30%;
}

/* ------------------------------------------------------ FORM-DESIGNS ---------------------------------------------- */
.form-main label {
    color: #B27EB3;
    margin: 0.5% 0%;
}
.form-main input {
    background: #f8e3f9;
    border: 1px solid #e5bde6;
    color: #B27EB3;
    margin: 0.5% 0%;
}
.form-main ::placeholder {
    color: #B27EB3;
    opacity: 1; /* Firefox */
}
.form-main select {
    background: #f8e3f9;
    border: 1px solid #e5bde6;
    color: #B27EB3;
    margin: 0.5% 0%;
}

/* ----------------------------------------------------- COLORS ----------------------------------------------------- */
.violet-color {
    color: #9f2ca2;
}
.red-color {
    color: #d3333c;
}

/* ----------------------------------------------------- NAVBAR ---------------------------------------------------- */

header a, header a:hover {
    text-decoration: none;
    color: white;
}
header {
    padding: 0 20px;
    background-color: #1d1f1d;
    height: 50px;
    display: flex;
    justify-content: space-between;
}
#brand {
    font-weight: bold;
    font-size: 18px;
    display: flex;
    align-items: center;
}
#brand a {
    color: white;
}
ul {
    list-style: none;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: space-around;
}
ul a {
    color: white;
}
ul li {
    padding: 5px;
    margin-left: 10px;
}

ul li:hover {
    transform: scale(1.1);
    transition: 0.3s;
}
#login, #signup {
    border-radius: 5px;
    padding: 5px, 8px;
}
#login {
    border: 1px solid #498afb;
}
#signup {
    border: 1px solid #ff3860;
}
#login a {
    color: #498afb;
}
#signup a {
    color: #ff3860;
}
#hamburger-icon {
    margin: auto 0;
    display: none;
    cursor: pointer;
    z-index: 999;
}
#hamburger-icon div {
    width: 35px;
    height: 3px;
    background-color: white;
    margin: 6px 0;
    transition: 0.4s;
}
.open .bar1 {
    -webkit-transform: rotate(-45deg) translate(-6px, 6px);
    transform: rotate(-45deg) translate(-6px, 6px);
}
.open .bar2 {
    opacity: 0;
}
.open .bar3 {
    -webkit-transform: rotate(45deg) translate(-6px, -6px);
    transform: rotate(45deg) translate(-6px, -6px);
}
.open .mobile-menu {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: flex-start;
}
.mobile-menu {
    display: none;
    position: absolute;
    top: 50px;
    left: 0;
    height: auto;
    width: 100%;
    background: black;
    padding-bottom: 5%;
}

/* ------------------------------------------------------ FOOTER ------------------------------------------------- */

#footer {
    height: 15vh;
    background: black;
    color: #387287;
    display: flex;
    padding: 2% 5% 0% 5%;
    flex-direction: column;
    text-decoration: none;
    font-size: 0.9rem;
    z-index: 5;
}
#footer_links {
    display: flex;
    justify-content: center;
    text-align: center;
    text-decoration: none;

}
#footer_links a {
    padding: 0% 10%;
    text-decoration: none;
}
#footer_links a p {
    text-decoration: none;
    color: #ffffff;
    opacity: 0.5;
}
#copyright {
    padding-top: 1%;
    display: flex;
    justify-content: center;
    opacity: 0.5;
    color: #ffffff;
}

/* ---------------------------------------------------- RESPONSIVE -------------------------------------------------- */
@media only screen and (max-width: 600px) {
    header nav {
        display:none;
    }
    #hamburger-icon {
        display: block;
    }
}