How can I save my styles' modifications for eternity in LibreOffice?
LibreOffice uses the settings of the default document template for a new file unless you select another template to start with. You want to substitute your own template for the default template.
Prepare a document containing your desired styles and settings, delete all the text and save the document as a template in the template management system (File/Templates/Save).
To make this document the default template, navigate on the menu File/Templates/Organize (may appear as File/Templates/Manage) then click on the template you made and click on the Commands button to bring up a drop down list. Select the Set as Default Template entry.
Each new document will now use that template until the default template is reset.
You can find out much about template use in Chapter 10 of the LibreOffice Writer Guide that is found at http://wiki.documentfoundation.org/Documentation
It is indeed extremely annoying that it doesn't seem possible to simply add one or more styles from your current document to the default template.
However, what is possible is to copy styles from one open document to another open document. In some situations, that may be sufficient as a workaround.
If you have a document open in Writer or in Calc, with the styles you need:
- assign the wanted style(s) to some empty lines or empty cells
- copy the empty lines or cells
- open the other document in which you want to have these custom styles
- paste the (empty) lines or cells into your other document.
Now, the custom styles from the first document will also be available in the second document.