Install obconf and/or lxappearance-obconf for a GUI to configure visual settings and theming.
A good selection of themes are available
in the openbox-themesAUR package or the AUR. Some GTK
themes come with an Openbox theme as well. Both Openbox-specific and Openbox-compatible themes will be
installed to the /usr/share/themes directory and will also be immediately available for
selection.
box-look.org is an excellent and well-established source of themes. deviantART.com is another excellent resource. Many more can be found online.
Edit or create
Tip: It is
better to copy a theme to your home directory than to edit those found in /usr/share/themes/. This will retain the
original should anything go wrong and ensure that your changes are not
overwritten on update.
The process of creating new or modifying existing themes is covered extensively at the official openbox.org website. obthemeAUR is a user-friendly GUI for doing so.