From Fluxbox-wiki

how to make windows borderless

There are two ways to achieve this. The easiest one is to add the following entry to your ~/.fluxbox/keys file:

  Mod4 t :ToggleDecor

Use the one above or choose another key combination (windows-key + t in this case).

You now have to reload the configuration with

  fluxbox menu -> reload config

or using a signal from the command line like so:

 kill -s usr2 `pgrep fluxbox`

After the reload your key combo is ready to work. The next step is to open the application you like to have borderless. Lets take an aterm as example.

Right click the window title to bring up the windowmenu. Go to the 'Remember' section and enable 'Save on close' and 'Decorations' Options. Then, use your keybinding to remove the window decorations. Closing the window will save the settings to your ~/.fluxbox/apps file. All aterms you open from now on will have no window decorations.

This way will be a global setting for one application. If you want for example your aterm with irssi running in it to be the borderless window BUT all other instances of the aterm show the window decoration normally, you need to set a name for this specific aterm. Most terminals use the "-name" switch for it:

   $> aterm -name irssiterm -e irssi

Applying the key-combination on that special aterm will create an entry in your apps file similar to this here:

   [app] (irssiterm)  
      [Deco]	{NONE}
   [end]   

and it will ONLY apply for an app named "irssiterm" which you can force via the -name switch.

The second way is to edit the apps file directly.

Open the Application you like run without decorations. Then open a term and type

  xprop WM_CLASS|cut -d \" -f 2

and click the application. This will give you the name to use in the apps file. Keep it in mind or write it down.

Open ~/.fluxbox/apps and add these lines:

  [app] (name of the app that you wrote down or kept in mind)
     [Deco]	{NONE}
  [end]

Save the file and reload the configuration from the fluxbox menu. The settings will now be used.