The package is severely out of date with respect to the Debian
Policy. Latest version is 4.7.0
and your package only follows
3.9.6...
The package has a debian/watch file, but the last attempt
to use it for checking for newer upstream versions failed with
an error:
In watchfile debian/watch, reading webpage http://download.gna.org/gtk-theme-switch/ failed: 500 Can't connect to download.gna.org:80 (Name or service not known)