I have been working with OpenGTS & GTS Enterprise (GTSE) for about 3 years and have 12 customers that I support on an ongoing basis.
I am quite familiar with the source code.... The UI code in OpenGTS is not a template or theme like for example Wordpress. It is scattered about the application in the XML config files, css, .jsp and .java files, and of course the images.
The work would be performed on your server in the $GTS_HOME directory, and the sub-directories under $GTS_HOME so you will have the source code.
Could you post an image of what you want the UI to look like, so I can get an idea of what is involved?
Also I have written several Device Communication Servers. Do you have a working Device Communications Server?
Kindest regards,
John H. Ward, Jr.