I just found out I can add custom keyboard shortcuts by going to System Preferences > Keyboard & Mouse > Keyboard Shortcuts, clicking plus, selecting the application, and typing in the corresponding menu name and keyboard shortcut I want. E.g., I can type in "Reopen All Last Windows From Session" to bind a keyboard shortcut for that command in Safari.
I want to know how to do this for a shortcut in a drop down menu. More specifically, I want to bind opening a window with a certain theme in Terminal to a shortcut (Shell > New Window > Theme Name).
Does anyone know how to do this?