Hello Mandar:

Goggle translate.

I give you three ways to do it.

1) a simple button.
2) a css styled button.
3) a button with css style and image.

<style>
/* 10 styles for buttons. */
.button{
width: auto;
height: auto;
background-color: #3f7d95;
border: solid 0px #ffffff;
color: #ffffff;
padding: 10px 10px;
text-align: center;
text-decoration: none;
display: inline-block;
font-family: "Open Sans", sans-serif;
font-size: 16px;
margin: 4px;
cursor: pointer;
border-radius: 9px;
transition-duration: 0.8s;
box-shadow: 0px 0px 0px 0px rgba(0,0,0,0.2), 0px 0px 10px 1px rgba(0,0,0,0.3);

/* max-width: 150px;
overflow: hidden;
text-overflow: ellipsis;
white-space: nowrap; */
}

.button1 {background-color: #3f7d95;}
.button2 {background-color: #fF2300;}
.button3 {background-color: #3f51b5;}
.button4 {background-color: #E91E63;}
.button5 {background-color: #23a2ab;}
.button6 {background-color: #9e6e9c;}
.button7 {background-color: #9e113a;}
.button8 {background-color: #06618f;}
.button9 {background-color: #1C7E5E;}
.button10 {background-color: #F19403;}

.button1:hover {background-color: #ff2300; border: solid 2px #ff2366;}
.button2:hover {background-color: #4CAF50;}
.button3:hover {background-color: #F19403;}
.button4:hover {background-color: #2CB67C;}
.button5:hover {background-color: #91ab41;}
.button6:hover {background-color: #99B62C;}
.button7:hover {background-color: #3f51b5;}
.button8:hover {background-color: #2196F3;}
.button9:hover {background-color: #E91E63;}
.button10:hover {background-color: #fF2300;}
</style>

<div>
<h4>Button simpli</h4>
<button onclick="window.close();">Close Window</button><br>

<h4>Buttons with style css</h4>
<a class="button button9" onclick="window.close();">Close Window</a><br>
<a class="button button5" onclick="window.close();">Close Window</a><br>
<a class="button button3" onclick="window.close();">Close Window</a><br>

<h4>Button with imagen</h4>
<a class="button button10" onclick="window.close();"><img src="https://www.radiomazz.com/files/Logo2021RM600x600px.png" alt="Close Window" style="width:100px;height:100px;"></a><br>
</div>

The buttons look like this:

Just choose the colors you like and change the code in the css style or choose the number with the color I assigned.

You know, the buttons go in an html object. and the css style goes in the advanced option of the html object. (if styles don't work for you, you'll have to put the style in the page properties before the closing head tag)

I assume that if you are going to use an image as a closing link, the image must mention it.

I assume you have the terms and conditions page hidden?
This should be 100% visible in the bottom menu.

I hope this is helpful to you.