def set_theme(self, theme_name):
package = theme_name if theme_name not in self.PACKAGES else self.PACKAGES[theme_name]<EOL>self.tk.call("<STR_LIT>", "<STR_LIT>", "<STR_LIT>".format(package))<EOL>self.tk.call("<STR_LIT>", theme_name)<EOL>
Set new theme to use. Uses a direct tk call to allow usage
of the themes supplied with this package.

:param theme_name: name of theme to activate