LXDE (Openbox): add "resize corner" to the window

I am using LXDE (Openbox). Every time I need to re-size my window, I have to position my mouse carefully to grab the thin window frame until my mouse cursor changes and I can re-size the window.

I remember seeing in other window managers a "resize corner" in the bottom right corner of the window, which can easily be grabbed to resize the window diagonally.

Does anything like this exist in LXDE (Openbox) ? How can I add it ? Can it be configured in ~/.local/share/themes/theme/openbox-3/themerc ?