It’s quite simple but you would have to be careful. You can change it from within zeroshell and not zerotruth.
I’ve been able to customize mine for my Hotspot.
Login to zeroshell admin page, goto Captive Portal–>Authentication and click on Template. a pop-up page will show you the customized template and the native template. dropdown the preview menu and click on view source. Here is where you can edit the code, but i WARN you ..be careful.. what i do is to copy the entire code before editing to notepad so that if there is any damage done, i simply revert back.
After editing, preview it and if you are ok, save it.