If a theme doesn't show up in lxappearance

You have 2 tools to configure the look and feel of Openbox : in the preferences of the openbox menu the gui config tool (Obconfig) and User Interface Settings (lxappearance) under Preferences.
If your desired theme doesn't show up in the latter, put it in /home/user/.themes and it will be found and can be activated. The smae with icons; create a folder .icons and install/put icons there.