Take the Word document, save it as a text file, load the text file into Sigil, create your CSS as you go about formatting/splitting the file into it's sections/chapter.
This way, you don't have to deal with the mess Word puts in and also fixing what Word put in. It's just going to be easier to create fresh styles and fresh formatting.
But before you do this, put a <p> at the beginning of every line/paragraph and a </p> at the end. Then Sigil will know where each bit is. Word can search/replace to do this.