How to customize the Journal outlook in OJS 3.3

Hello Everyone!
I am using OJS for last couple of months for Academic Journal purpose. I would like to customize the existing theme by myself, like to add more beautification. But I could not figure out the proper backend where I could make the changes. I have little knowledge on html and css. By inspecting the page, the proper place for modification can be identified but could not find the proper backend file in public_html.

I have studied on same topics from the forum. But do not have the proper clarification about customizing the theme.
Can any one provide support regarding this issue?


a good start is the Theming Guide.