Do the above changes before you go online and start getting registered users. Why? Because if you already have some, you don't know if the theme they chose in the User Preferences was the one you
want to be the default. If it is not, and you apply either solution above, then those users will not be able to get into your site! Either the theme they specified will not be there, or they will not
be able to change it.
The first solution has the extra disadvantage that if you choose to change the default theme, you will need to move it under the themes folder. Thus, even for a very short period of time, you will
end up having two themes available. You never know if a user just chose the theme you wanted to remove as her default theme. That user will be denied access to your site the next time she tries,
because the theme will not be there anymore!
In cases you do have registered users, you are thus advised to run the following SQL statement, either from the MySQL prompt, or through a graphical frontend, like phpMyAdmin:
update nuke_users set theme=";
This will set the theme to blank for all users, making it for them possible to change it to the default one afterwards, see disabling themes and also Where is the themeheader function located?.