First of all, if you using Edubart's OTClient which you have source to, you should follow me
About
Client Window Name, you should look into
src/main.cpp
for line's:
C++:
// setup application name and version
g_app.setName("Client Name"); // Name of Client
g_app.setCompactName("name of process in task manager"); // Name of process in Task Manager
g_app.setVersion(VERSION);
About changing login screen you must take a look into
modules/client_entergame/ and here you need to set-up everything for your own.
Little tip:
.otui files are files that define style to windows/labels with texts/other UI things (that user of client see on screen), basically graphics (position of windows, formatting, etc.).
If you want to know something about OTC, you need to start practice, do practice and keep practicing and try, try, try things by your own.
Editing OTClient is a long way my friend and it's basically consuming much time and effort.
Good luck,
Hope I help in something.
Peace! o/