To change the background you want:
If you haven't already, create empty file custom.css in folder /templates/purity_iii/css/ then add to it following selector and adjust it to your liking:
background: url('your_image_path') no-repeat scroll 0 0 transparent;
Regarding that YouTube video:
You are already looking at ...
T3 should be loading the custom.css file last.
As a work around, try EasyScript which is free and which I have used successfully for this purpose.
An important advantage of using EasyScript or similar over a core hack is that your changes will survive future updates.
Hello Haris and welcome to JSE. The biggest problem you are going to have if you modify any of the template files in order to change the load order is that updating your template in the future will revert your changes. A easy way to accomplish this would be to add a style declaration before the page finishes loading.
Save your custom CSS file to the root of ...