Apparently, you format your text using buttons in the toolbar to get “locally” the effect you expect to deviate from the default format.
Workflow in LO Writer is based on the use of styles. Styles can be seen as a kind of semantic mark up for paragraphs (at first level). With styles, you “declare” paragraphs as being heading, main discourse, comments, explanation, argument, nbote, … Then each specific style can be given graphical attributes (idents, vertical spacing, justification, font face and size, …) to render the semantic difference.
Character styles are a second level to distinguish words with another meaning inside a paragraph through a supplementary markup.
This neat organisation is defeated by “direct formatting”, i.e. forcing attributes with toolbar buttons (or shortcut equivalent), because it overrides style attributes. It introduces a semantic bias in the document markup. Direct formatting should be reserved for experimenting to test rendering.
In your case, if you did not clearly attribute different styles for different semantic-significance paragraphs (e.g. working with the default paragraph style and adjusting “effects” with buttons), you have no other choice than going on with direct formatting and manually fix each instance.
If so, try to review your document style-wise. This is longer than direct formatting but this will ease future editing(where you would only play on styles instead of text).
