[openbox] How to get a thick border around focused window?

[chombee at lavabit.com]

How can I get openbox to draw a thick, coloured border around the
currently focused window? Right now if my windows are undecorated then
there's no visual indication of which has focus. With some themes even if
they're decorated  there isn't. I'm guessing this is a theme thing not a
config thing? Recommend a theme? I keep most windows undecorated and just
want a coloured border to identify the focused one, in the style of dwm
and other window managers that don't draw traditional window title bars
with buttons etc.