See this
There are 3 elements to a theme (but only the first two are required): page templates, styles and scripts.
If you copied an existing theme, templates will be found in templates folder. They are html documents with additional benchpress templating syntax and they define the bare layout and contents of the page. You can find some basic information on templates here and there is also benchpress documentation that will probably be useful.
Styles are just less css files (located in less folder usually) and define the look of the page (and can also define layout). You can learn more about less here or just by googling some tutorials 
As for JavaScript - unless you want to do something specific with the way the style behaves, whatever you copied will probably be fine. Server-side you've got mostly widget area definitions, while client-side there usually is just the loading bar. But if you want to you probably can extend that further easily
@oplik0 said in How to customize the Theme's design?:
Thank you for your reply.
I want to know where to write the code for design and I don't need follow components.
Instead of this, I need to add the following screen.
You can check this image on http://planetstyles.net/demo/milk/3.3/1/viewtopic.php?f=5&t=25&sid=834ed09ad81c0c19778c16063ed453df
Please teach me.
Thank you again.
