Problem with modal

I am trying to install different modal on my website. I tried using plug or using CSS. Both are not working as I want them to:
– for the plugin it doesn’t work at all, nothing happen and I tried different plug in so I think the problem is from the Theme
– using the CSS, when we click on the link the open the modal, the modal go to the top of the page and didn’t go back when we close it.

I would like to use CSS, I think it is easier. Do you have any idea about how I can do?

My website where there is the modals: https://uberis.duponddupont.com/about-us.

My CSS code:

/* Fenêtre settings MODAL */
@-webkit-keyframes example {
	from {
		top: -100px;
		opacity: 0;
	}

	to {
		top: 0;
		opacity: 1;
	}
}

/* Add animation (Standard syntax) */
@keyframes example {
	from {
		top: -100px;
		opacity: 0;
	}

	to {
		top: 0;
		opacity: 1;
	}
}

/* The modal's background */
.modal {
	display: none;
	position: absolute;
	left: 0;
	top: 0;
	width: 100%;
	height: 100%;
	overflow: auto;
	background-color: rgb(0,0,0);
	background-color: rgba(0,0,0,0.4);
}

/* Display the modal when targeted */
.modal:target {
	display: table;
	position: absolute;
}

/* The modal box */
.modal-dialog {
	display: table-cell;
	vertical-align: middle;
	text-align: auto;
	width: 70%;
}

/* The modal's content */
.modal-dialog .modal-content {
	margin: auto;
	background-color: #f9f9f9;
	position: relative;
	padding: 0;
	outline: 0;
	border: 1px #777 solid;
	width: 70%;
	box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2), 0 6px 20px 0 rgba(0,0,0,0.19);
/* Add animation */
	-webkit-animation-name: example;
/* Chrome, Safari, Opera */
	-webkit-animation-duration: .5s;
/* Chrome, Safari, Opera */
	animation-name: example;
	animation-duration: .5s;
}

.header {
	color: black;
}

/* The button used to close the modal */
.closebtn {
	text-decoration: none;
	font-size: 35px;
	font-weight: bold;
	color: grey;
	text-shadow: 1px 2px 3px rgba(0,0,0,0.5);
}

.closebtn:hover,
.closebtn:focus {
	color: #000;
	text-decoration: none;
	cursor: pointer;
}

.container {
	padding: 2px 16px;
	width: auto;
}

header.container {
	background: #f3f3f3;
}