Paragraph style changes don't save consistently

I added a personal style to LO, but the font changes every time i try to use it in new document (Ariel to Times New Roman)

You have to make the changes in the underlying template, usually the Default template.

If its Paragraph style you are speaking of, then try pasting at the first character of new para. The insertion point should not be inside some other para. In short, start a new para, not characters on that, paste there. While copying, select the whole para to which the style you want to copy is applied. Partial selection may cause some changes.