Add a simple html css modal in theme.liquid

[sneh]

Hello this is Sneh.

I am having issues while adding a simple html css modal to the theme.liquid file.
How can I add this on my website?

This is the html code:

<script type="text/javascript">
// Removes the HTML dag div with the class popup from javascript using in-built html-JS methods
function myFunction() {
document.getElementById("cls1").innerHTML = "";
}

</script>

<div id="cls1">
<div class="popup">
<h2>Pop-up name here</h2>

<button class="close" onclick="myFunction()">&times;</button>

<div class="content">
Pop-Up text here.
</div>

</div>
</div>

.popup {
padding: 55px;
background: #72C6D5;
border-radius: 5px;
width: 30%;
position: fixed;
bottom: 0;
right: 0;
transition: all 5s ease-in-out;
}

.popup h2 {
margin-top: 0;
color: #333;
font-family: Tahoma, Arial, sans-serif;
}

.popup .close {
position: absolute;
top: 20px;
right: 30px;
transition: all 200ms;
font-size: 30px;
font-weight: bold;
text-decoration: none;
color: #333;
}

.popup .close:hover {
color: #06D85F;
}

.popup .content {
max-height: 30%;
overflow: auto;
}

@media screen and (max-width: 700px){
.box{
width: 70%;
}
.popup{
width: 70%;
}
}

This is the css: