Themes - I don't like or want the default theme - can't I just change it
From Online Manual
While it is possible to make changes to the default theme, this is not recommended. This is because it could cause problems for custom themes. Custom themes often expect to find files in certain places in the default theme. If it they cannot be found, or if they are sufficiently different compared to their original form, you may find that some of your custom themes do not work, or do not work correctly. Making changes to the default theme could also cause problems when you install some mods you might like to have on your forum.
If you decide to make changes to the default theme, make sure that you have a backup (either from your own forum or from the SMF install), just in case your changes cause problems for your forum.