Ideally, you don't need any customizing at all. But if you really want to customize FreeMind, copy the file "user.properties" to the folder "freemind" in your home directory and edit the properties in the text file using your favorite text editor, e.g. notepad, Vim or Emacs.
You certainly know your home directory if you're working on Linux. On Windows 2000, it's typically "C:\Documents and Settings\<UserName>\", on Windows 95, it's "C:\WINDOWS\". Edit "user.properties" in the folder "freemind" of your home directory.
On Windows, you can additionally configure files with the extension .mm to open with FreeMind.
For details on editing options, see The user.properties file. For details on editing patterns, see Physical style - patterns.