It looks like this is not documented anywhere.
Yet, looking at the source code it appears you have a small set of options to perform minor cosmetic customizations:
Taken from the source code:
function completely(container, config) {
config = config || {};
config.fontSize = config.fontSize || '16px';
config.fontFamily = config.fontFamily || 'sans-serif';
config.promptInnerHTML = config.promptInnerHTML || '';
config.color = config.color || '#333';
config.hintColor = config.hintColor || '#aaa';
config.backgroundColor = config.backgroundColor || '#fff';
config.dropDownBorderColor = config.dropDownBorderColor || '#aaa';
config.dropDownZIndex = config.dropDownZIndex || '100';
config.dropDownOnHoverBackgroundColor = config.dropDownOnHoverBackgroundColor || '#ddd';
However, it's worth nothing that you can access any of the HTML wrapped elements so you could just do anything yourself.