I have a LO installed on a MX Linux 21.3 (Debian 11 based OS), with LO 7.0.4.2, I have a document writtern with styles and would like to change ONLY fonts, when I tried to highlight the whole document using Crtl+A, the character is grayed out. Is there a way to change characters easily (keeping all other formats, such as bold, italic, indent, etc.).
In the sidebar at the right, click on the Styles icon. Right click the relevant paragraph style, select Modify and choose the new font, OK. All paragraphs using that style with display the new font
Your problem description is not crystal-clear.
This is the favourable case provided you really used styles exclusively, i.e. you have no direct formatting. This means that “highlighted” words inside paragraphs also styled with paragraph styles. Then what you need to do is to change a few styles.
Paragraph styles
Paragraph styles are structured in a tree-like organisation. Styles down in the tree inherit from styles higher in there. You can display the structure by choosing Hierarchical in the bottom menu of the style sidepane (F11). You’ll see that all styles derive from Default Paragraph Style.
Theoretically, modifying Font in Default Paragraph Style should be sufficient.
There is a more subtle way to do this since there are some overrides in the factory settings of styles. Go to Tools>Options, LibreOffice Writer>Basic Fonts (Western). You can change there the font face for several sub-trees in the style hierarchy. This will work if you didn’t change font face directly in styles Default Paragraph Style, Heading (all headings), List, Caption and Index (for TOC and other “Table of …”).
If you changed font face in any style, this style and its descendants won’t be affected by the changes you make. You’ll have to modify the overridden styles individually, eventually pressing Reset to Parent so that it inherits again.
Character styles
They can also be organised as a tree. As configured in factory, there is no central character style (they are all independent from each other). Consequently, if you forced a font in a character style (by default, none except Source Text, Teletype and User Entry, change the font face), you must either change it individually or erase the override with Reset to Parent.
Direct formatting
This is evil. Any direct formatting must be fixed individually.
The best thing you can do is to replace direct formatting with adequate paragraph or character styles. Once this is done, select your whole text and Ctrl+M (or Format>Clear Direct-formatting). This is necessary because direct formatting takes precedence over paragraph and character styles.
That’s my trouble, I don’t remember, I made the changes after learning new features/tips, so I kept adding them, I’m afraid that it’s a mess, that’s why I’m thinking in changing only fonts and not through styles, it seems I have no choice but to do it through styles.
If you want to change the font for different paragraph styles, then you might only need to Modify the Default Paragraph Style for the change to cascade down through the inheritor styles