Save the existing Theme
You may use every existing theme as a base to build your own customized theme.
Every theme has its own directory: Cloudy, Xenia, ..., with all the necessary files inside.
Before you make changes, copy the directory (with all files inside) from the theme you would like to change. Store this copy using a new directory name (for example "MyNewTheme"). Then you can feel free to change anything you like, to build your own theme and to customize the code.
Editor to Change Code
To make changes in the code, it will be very helpful to use a specialized editor!
Notepad++ works well, open-source like PGV, and is free.
See the How To:Build a New Menu tutorial.