The default theme is part of the SMF installation which contains the base templates and language files that will make the forum display correctly. Modifying the default theme or deleting files that are part of it will make it difficult to upgrade, install, modify, or add more themes in the future. It is strongly recommended to keep the default theme in one piece, so that if a file is corrupted or missing from another theme it will be possible to revert to the default theme.

If it is necessary to modify the default theme the best way is to make a copy and modify it. If something is incorrect, or missing in the modified theme the forum will not be affected in any way. The reason is modified themes often use templates or language files that the default theme has, and if a template or a language file is missing or corrupted, SMF will look for these templates in the default theme.

Keep in mind that images and style sheets from the default theme are not part of the fall back system. The purpose of the fall back system is to make the theme development easier. For theme development please refer to Creating and Understanding Themes.