-- To create a new theme --
1: Duplicate the 'default' theme folder and rename it to the name of your theme.
2: Upload your page templates and file assets into this new theme folder
3: To change the active theme go to the Theme page in the admin area

If a particular file cannot be found in a custom theme the CMS will try to fall back and use a corresponding file from the default theme.
Therefore it is recommended not to edit or remove any files in the default theme.
Instead create a new theme folder as outlined above and make any edits to the files in the custom theme.