* { -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;
}

/* ----------------------------------------------------- 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%;
}

/* --------------------------------------------------------- TEXT --------------------------------------------------- */
.text-center {
    text-align: center;
}
.text-left {
    text-align: left;
}
.text-right {
    text-align: right;
}
.text-bold {
    text-size: .75rem;
    font-weight: bold;
}
.text-medium {
    text-size: .75rem;
    font-weight: 600;
}
.text-normal {
    text-size: .75rem;
    font-weight: 400;
}

/* -------------------------------------------------------- HEADINGS ------------------------------------------------ */
.heading-medium {
    font-size: 2rem;
    font-weight: 400;
}
.heading-small {
    font-size: 1.5rem;
    font-weight: 300;
}
.heading-large {
    font-size: 2.5rem;
    font-weight: 400;
}
.heading-extra-large {
    font-size: 3.5rem;
    font-weight: 400;
}

/* --------------------------------------------------------- BUTTONS ------------------------------------------------ */
.btn-1 {
    background: black;
    color: white;
    border: none;
    transition: all 0.15s ease-in-out;
    padding:1% 0%;
    width: 100%;
    text-decoration: none;
}
.btn-1:hover {
    background: black;
    transform: scale(0.99);
    color:white;
}
.btn-1 a {
    text-decoration:none;
}

.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);
}
.complete-profile {
    font-size: 2rem;
}
.whatsapp {
    width: 80%;
    background:green;
    text-decoration: none;
}
.whatsapp:hover {
    background:#2a5e14;
    text-decoration: none;
}
/* --------------------------------------------------- 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: 80%;
}
.default-form-container {
    width: 100%;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
}
.default-form-container form {
    display: flex;
    flex-direction: column;
    width: 30%;
    padding:1% 0%;
}

/* ------------------------------------------------------ FORM-DESIGNS ---------------------------------------------- */
.form-main label {
    color: black;
    margin: 0.5% 0%;
}
.captcha-img #captcha_input {
        width:48%;
        background: none;
        border: 3px solid grey;
        color: black;
}
.captcha-img {
    display:flex;
    width:100%;
    padding-bottom:1%;
    justify-content: space-between;
}
.captcha-img div {
        width: 50%;
    }
.form-main input, textarea {
    background: none;
    border: 3px solid grey;
    color: black;
    margin: 0.5% 0%;
}
.form-main ::placeholder {
    color: grey;
    opacity: 1; /* Firefox */
}
.form-main select {
    background: #f8e3f9;
    border: 1px solid #e5bde6;
    color: #B27EB3;
    margin: 0.5% 0%;
}

/* ------------------------------------------------------ DIV ------------------------------------------------------- */
.full-page-div {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 90vh;
    width:100%;
}

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



/* ------------------------------------------------------ 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;
}
/* --------------------------------------------------- FLASH MESSAGE ------------------------------------------------ */
.success {
    color: green;
}
.error {
    color: red;
}

/* ---------------------------------------------------- OTHER CSS --------------------------------------------------- */

/* ------------------- RETREATS ---------------------*/
.retreat-container {
        width:100%;
        display: flex;
        flex-direction: column;
        align-items: center;
        justify-content:center;
    }
    .retreat-container img {

    }
    .retreat-intro-box {
        position:relative;
        display:flex;
        flex-direction:column;
        background: white;
        margin-top: -10%;
        width: 60%;
        align-items:center;
    }
    .heading-medium {
        padding-top: 2%;
        margin-bottom:0%;
    }
    .text-bold {
        padding-top: 5%;
    }
    .retreat-venue {
        width: 50%;
    }
    .retreat-activities {
        display:flex;
        flex-direction:row;
        flex-wrap:wrap;
        width: 100%;
        align-items:center;
        padding: 2% 10%;
        justify-content:center;
    }
    .retreat-activities-card {
        width: 30%;
        height:100px;
        background: #EBEBEB;
        padding: 1% 2%;
        box-shadow: 3px 3px 3px rgba(0, 0, 0, 0.2);
        margin: 1.5%;
    }
    .retreat-activities-card-heading {
        font-weight: 500;
        opacity:0.7;
        margin-bottom:0;
    }
    .retreat-activities-card-text {
        font-weight: 400;
        padding-top:0;
        opacity: 0.7;
    }
    .retreat-prerequisites, .retreat-accommodation {
        background:#EBEBEB;
        width: 100%;
        text-align: center;
        padding: 2% 20%;
    }
    .retreat-prerequisites-text, .retreat-to-bring-text, .retreat-accommodation-text {
        font-weight: 400;
        opacity: .7;
    }
    .retreat-to-bring {
        background:#FFFFFF;
        text-align: center;
        padding: 2% 20%;
        width: 100%;
    }

/* ---------------------------------------------------- RESPONSIVE -------------------------------------------------- */
@media only screen and (max-width: 600px) {
    header nav {
        display:none;
    }
    #hamburger-icon {
        display: block;
    }
    .heading-large {
        font-size: 1.5rem;
        font-weight: 400;
    }
    .artwork-img-container img {
        width: 100%;
        padding: 0% 2% 2% 2%;
    }
    .default-form-container form {
        width: 80%;
    }
    .retreat-intro-box {
        width: 80%;
    }
    .retreat-activities {
        padding: 2%;
    }
    .retreat-activities-card {
        width: 80%;
        height: 75px;
    }
}
@media only screen and (min-width: 600px) {
    .full-page-form-container form {
        width: 70%;
    }
    .whatsapp {
        width: 70%;
    }
    .heading-large {
        font-size: 1.7rem;
        font-weight: 400;
    }
    .default-form-container form {
        width: 50%;
    }
    .retreat-intro-box {
        width: 80%;
    }
    .retreat-activities {
        padding: 2%;
    }
    .retreat-activities-card {
        width: 45%;
        height: 75px;
    }
}
@media only screen and (min-width: 900px) {
    .heading-large {
        font-size: 2.1rem;
        font-weight: 400;
    }
    .default-form-container form {
        width: 40%;
    }
    .retreat-intro-box {
        margin-top: -10%;
        width: 60%;
    }
    .retreat-activities {
        padding: 2% 10%;
    }
    .retreat-activities-card {
        width: 30%;
        height: 100px;
    }
}

@media only screen and (min-width: 1280px) {
    .full-page-form-container form {
        width: 50%;
    }
    .whatsapp {
        width: 50%;
    }
    .heading-large {
        font-size: 2.2rem;
        font-weight: 400;
    }
}
@media only screen and (min-width: 1920px) {
    .full-page-form-container form {
        width: 40%;
    }
    .whatsapp {
        width: 40%;
    }
    .heading-large {
        font-size: 2.3rem;
        font-weight: 400;
    }
}
@media only screen and (min-width: 2500px) {
    .full-page-form-container form {
        width: 30%;
    }
    .whatsapp {
        width: 30%;
    }
    .heading-large {
        font-size: 2.5rem;
        font-weight: 400;
    }
}