There are two ways. First, using the CTRL-H dialog to get the “Find and Replace” window, then choose “Format”, and select the attributes you want from the next window. You might need to toggle-on the “Other Options” drop-down to get these buttons:
Your other option is to use the “Properties” option in the very useful LibreOffice extension, Alternative Find & Replace (you’ll need to install it, of course):
If you search AskLibO for “alternative find & replace”, you’ll find quite a few Q&As dealing with it. Very useful!
2. Characters following return?
You need to use the “Regular expressions” option with CTRL-H; you might need to toggle the “Other Options” drop-down to see it (see the first pic, above).
For characters that follow a return (i.e., first characters of next paragraph), search like this (assuming you’re looking for “cat”):
^cat
That ^ signals “match at beginning”. To get “cat” followed by a return, search for:
cat$
and $ signals “match at the end”. (So searching for ^$ should find you all “empty” paragraphs.)
3. Insert return?
Using the “Regular expressions” option, search for cat, and then for “Replace with”, use cat\n, and the \n will add the “new line” or ¶ for you. (Don’t forget to uncheck “Regular expressions” when you’re done, or you may be mystified next time you search for something, forgetting that you’re using regex! Been there, done that.)
Also, do look at other LibO find-and-replace Q&As for plenty more of value. Hope this helps for a start!