open the terminal then type

>gnome-keybinding-properties

Then an window will pop up that window can be used to create custom keyboard shortcuts.

Click add and give name for the entry and terminal command which should be activated with shortcut and click Apply

now in the window backward an entry such as custom entry will appear and now click in shortcut then the disabled option changes to New shortcut .now press the needed shortcut it will be set.

command for terminal is gnome-terminal and give needed shortcuts