How can I customize WebViewer's UI in Appian?

You can use the enabledElements and disabledElements parameters (list of text) to add or remove UI elements. To find the name of an element, right click on it (f.e. right click on a button) > then select Inspect element - this will open your browser console with the html structure of the current page. Look for the data-element html attribute and pass it to one of the above parameters. You can review the steps in this video here.