After I got bored with the default font families and font sizes of the Default, Text Body and Heading styles, I tried changing them permanently. But just pressing F11, modifying certain styles and Applying isn’t enough for Libre Office to remember your specific choices.

No, what you actually have to do is apply your personal changes to the default styles in any Writer document, then delete all contents and go to File - Templates - Save Template. This file will be saved as a template for future use as a default template, but you need to make it default first.

Go to File - Templates - Organize. Double Click My Templates and select the template you saved just seconds ago. Click the Command button and click Set Default Template.

Now close Writer and reopen it. If you look at the styles by pressing F11 and modifying them, you’ll see they are exactly as you changed them in your previous document, and they will stay that way for eternity!

The actual source of this solution is found here, but I just saved you the trouble of looking even further.