It is actually very easy - just create a folder named “webuires” in your game’s source directory, copy here all the files you need to customize from the same named directory in library and modify these files as needed, adding any other images, css, js files. Then specify webuires as a resource.
For example I’ve created a library file extensionChat.tl:
name: Multiplayer Chat
resource: webuires and my webuires folder contains: cmdwin.js chatwin.css chatwin.htm chatwin.js layoutwin.js main.htm main.js, some of which are new files and others are slightly tweaked conterparts of original from library. Next I’ve included my extension with -lib extensionChat in makefile. That with frob on linux, but it should be similar in workbench.
What Jerry cited is valid in general case, useful for example when generating some responses dynamically from within TADS code, but everything which reside in /webuires/ path is automatically treated and served as a static resource so no need to worry.