I entered the above but didn't change it.

<!--Top button -->
<script src="./topbutton/jquery-1.11.3.min.js"></script>
<script src="./topbutton/topbutton.min.js"></script>
<script>
$(document).ready(function(){
//Top Button
$.topbutton({
htlm : "<i>Top</i>", //String
css : "width:50px; height:50px; background color: #343a40 = rgb(52, 58, 64); border:false; font-size:20px;", //String
scrollAndShow : true, //Boolean
scrollSpeed : 1000 //Number
});
if ($('div.wrapperChat').length)
$('div.wrapperChat')
.closest('#bx-content-container')
.parent()
.css('height', 'calc( 100vh - 9rem )');
});
</script>
<style>
.bx-tl-post .bx-form-value .bx-form-input-wrapper-textarea {
overflow: unset;
}
</style>
<!--End Top Button -->

Because you are editing the wrong file!

#343a40 = rgb(52, 58, 64) <- It doesn't have to be put anywhere!!!

Only:  #343a40 or rgb(52, 58, 64) 

In your case .. the required color code!