It depends. Some portions of most themes should be customized from the Admin panel. Those settings are stored in the WordPress database.
Things that cannot be customized from the Admin panel require changes to CSS and php files, which is best done on your own desktop computer and uploaded to the web site. Offline editing, as you put it.
For that offline editing, a syntax highlighting editor is a big help. NotePad++ is my personal favourite. Never use a word processor like Microsoft Word as it does not create standard text files, which is what you need.
You only need a text editor to change things offline. But I’m sure you want to see your changes as well, so you’ll need to install the whole server environment as well. MAMP or WAMP, depending on whether you’re using Mac or Windows at home, will help you get started with that.
Dreamweaver isn’t necessary.
“You only need a text editor to change things offline.” NotePad++ can also be called through FileZilla’ View/Edit feature to simulate direct editing of files on the host.
In general, it is good practice to change files locally rather than on your web host, especially in conjunction with some sort of versioning so you can “undo” a failing change.
There are just too many examples of folks who have got themselves in big trouble using the Editor within WordPress to do major changes to Themes and Plugins.
