How to change the default meaning of normal margins

I would like to know how to change what Normal margin means on LibreOffice. The options are Narrow, Normal and Wide and I would like Normal to mean 2.5 cm on every side and not 2.0 cm. I don’t want to create templates, just have a different meaning of Normal.

Edit: I want that if I choose normal on any document the 2.5 cm margin is applied.

Margins options

These icon-driven settings are conveniences for newbies, in the spirit of M$ Word ribbon UI. Learn how to use page styles and tune them for your own needs.

Also read the following tutorials to understand page styles:
https://forum.openoffice.org/en/forum/viewtopic.php?f=71&t=44607
https://forum.openoffice.org/en/forum/viewtopic.php?f=71&t=1221

And, by the way:

I don’t want to create templates

If you need to use the same page settings for many/all new documents, you have to create at least one new template and set is as the default one.