Strangely, by default, there is no toggle to decorate/undecorate a window.

You need to do one of two things: either define a keybinding to toggle or use the shortcut key to redisplay the window menu.

Lets examine option 1

To do this:

1. Open lxterminal and run:

   
   

   leafpad ~/.config/openbox/lubuntu-rc.xml
   
   

   
   



2. Search for the first instance of <keybinding>.

   
   



3. Then add the following keybinding:

   
   

   

   
   

   <keybind key="W-A-D">
   
     <action name="ToggleDecorations"></action>
   
   </keybind>
   
   

   
   



4. Now run:

   
   

   openbox --reconfigure
   
   

   
   

   to make your changes effective (alternatively, you can logout and login).

   
   



5. You'll be able to toggle the decoration using Meta+Alt+D.

   
   

   The Windows key is also another name for Meta.

   
   

Lets also examine option 2

Thanks to Glutanimate.

Pressing Alt+Space will display the window submenu where you can select Un/Decorate. Obviously this doesn't need a keybinding, but it's nice to have a toggle option to quickly turn the titlebar on/off.