I have a jar file called ScreenCapture.jar
I use http://jd.benow.ca/ what is in there. I have even downloaded jd-gui
This is what it shows me
follow these steps:
Save the source files by clicking save menu item under file. It will generate src.ip file.Then you can do whatever you want.
On your first question JD-GUI is just a viewer you need to use dex2jar to retrieve the JAR file; decompile and save the classes you need to change; import JAR and JAVA files in your Android Studio; modify; recompile and repackaged.