Koha OPAC Customization is very simple process in latest version of Koha 19.11 above.

1.1. Change Banner of Koha OPAC Header:


1. First of all, we will add Banner Image in Header Section.

2. Create a OPAC Banner Image header.png 

3. Copy this image into htdocs folder of koha Server.

4. Put your header image in koha home, and open terminal apply the command.
sudo cp /home/koha/header.png /usr/share/koha/opac/htdocs
(OR)
   
  Free image hosting Uploading your images

Go to:  Tools › News › New entry 

1. Now go to Tools › News Click on New Entry and select opacheader (en)

2. Enter values as showing in below image.

Koha Server Image Path.
<header><img src="/header.png" alt="" width="1230" height="190" /></header>

Private Image Hosting.
<header><img src="https://i.ibb.co/Yprx9fx/header.png" alt="" width="1230" height="190" /></header>

1.2. Change Koha OPAC Footer:

Go to:  Tools › News › New entry 

1. Now go to Tools › News Click on New Entry and select opaccredits (en)

2. Enter values as showing in below image.
Copy and Paste the following HTML. (Source Code Section)
<footer>
<div class="container">
<center>
<span style="color: #17202a;">
<br/>Designed &amp; Maintained by:<strong> 
<a href="#" target="_blank" rel="noopener">Lib Power Tech</a>
</strong><br/>Central Library, <strong>Contact: <span style="color: #2dc26b;">sivamdeveloper</span>
<span style="color: #2dc26b;">@gmail.com</span><strong><br/>
<br/></strong></strong></span></center></div>
</footer>