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