I just applied one of these themes: How can I set a dark theme in TeXstudio?
How can I switch back to the stock theme?
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The default settings are built in. They apply if no changes are made to the respective format. Format changes are part of the general settings in texstudio.ini.
To remove all changed formats, simply delete the section
[formats] in texstudio.ini (Make sure TXS are closed while you do so, otherwise your modifications won't take effect and will be overwritten when TXS closes).