If you create something, then you (or your employer) owns the copyright to that thing. Copyright basically means exactly what it says, only you have the “right” to make “copies” of it. Simple, really.

A license is basically you, the copyright holder, saying what other people (licensors) are allowed to do with the work. If you (or your employer) are not giving the work to other people, then you don’t need to worry about the license.

It is quite likely that the GPL, being a copyleft license, will indeed apply to your theme. But all of its provisions only matter when the theme is distributed to others. Without that, it does nothing on its own.