Welcome to the forum....and welcome to the world of REALLY BAD html...
I am not familiar with Libre Office...maybe someone else can help there. In general you might try going back into Libre and selecting all of the text (ctrl-a) and then make them all the same size and style and color of font...just keep the bold and italics. Then see if it will allow you to save in a "clean html" or a "filtered html" like Word. If you can do that, it might clean up a lot of that mess.
Another option is to get very familiar with the search and replace functions of Sigil. you will need to find all of the extra tags and replace them with (nothing). You shouls only leave a <p> at the beginning of each paragraph, and a </p> at the end - and the tags for bold and italics. After you have it cleaned up you can select each of your chapter titles and make them a header - click on the "h#" key in sigil. That will leave you with a <h> and an </h> instead of <p></p>.
For the fancy stuff you can go to W3Schools
where they have an excellent tutorial for html and css.