To add a new theme, simply create a new directory in the electrum data directory (either ./data or your system wide local data).

Inside your directory create a file called name.cfg with the name of that theme.

Create another file called style.css - this will be your CSS for the theme (see other themes for reference).

Documentation on Qt's stylesheets (used by Electrum):

Overview: http://qt-project.org/doc/qt-4.8/stylesheet.html
Examples: http://qt-project.org/doc/qt-4.8/stylesheet-examples.html
Reference manual: http://doc.qt.nokia.com/4.7-snapshot/stylesheet-reference.html