Please send theme files with or without a screenshot to UltraEdit support by email for the themes page. That's better than uploading an UltraEdit theme as attachment to a forum post as the theme will be not seen by users except they are explicitly searching for it which is a problem when name of theme not known.

Customizable is only what Windows allows applications to customize - the inner area of the window. The main window with the title bar and the window border itself is still drawn by Windows according to the Windows desktop theme and is therefore identical for all windows (except Outlook 2010 and some other applications). UltraEdit has no setting to hide the main application window. There is only the full screen mode.

For developers take a look on the Microsoft documentation page About Windows and the documentation page about the function GetSystemMetrics.