If something goes wrong while editing an SMF (Simple Machines Forum) theme you can be left without a way to access the forum, for example due to a php error you inadvertently introduced.
The best practice would be to never modify or use the default SMF default theme but make a copy of it and use it for the forum. This way you always have a theme which should work.
You can force the forum to use a different theme for your session by specifying a theme # in the url like below:
http://www.example.com/index.php?theme=1