To add to what DaleDe said, there is a difference between style sheets (css) and in-line styles. If you are faced with the latter, as you seem to suggest (there being no need to fuss with the styles if you're simply replacing a style sheet), then it's basically a matter of using the Find/Replace function in Sigil to get rid of a long, involved paragraph style and to replace it with a simple
<p> which will default to your basic paragraph style. Here is mine:
p {
margin-top:0.0em;
margin-bottom:0.0em;
text-indent:1.5em;
text-align:justify;
}
Which is to say: a simple, justified paragraph with a modestly indented first line and no extra space between paragraphs. You can see (and borrow, if you like) my style sheet at
http://notjohnkdp.blogspot.com/