You can put them in existing or new .lib files (if you use new ones you just need to add the references in the source)
Depending what UI changes you are making, you need to look in the various dialog.cs within the source code.
One big thing that helped a lot was one of my team seperated the maindialog into seperate .cs files to manage each part (so belt, char page etc) rather than 1 huge .cs