I used a linear gradient onto a line and now it applies to every new object I create. How can I stop this?
If you would like new objects to all start with the same standard appearance, rather than the last used appearance, click the Appearance Panel Menu and ensure
New Art Has Basic Appearance is checked.
Then, anything newly created will have a basic standard appearance. You can always go back and uncheck that option if you should ever want new art to retain the last appearance again.