Hi Vishal,
Changing the theme can be done in two places: css styling can be found in the themes/Default/main.css file and the <code class="CodeInline">resources/templates folder contains html templates written in Twig syntax. You can customize these by placing a modified copy in your themes/Default/templates` folder.
If you do change the footer template, please help us by keeping any of the copyright, credit or link information from the bottom. This information is vital for our project in a number of ways.
Thanks!