@arethusa
If you have different styles for some paragraphs, you must edit the html file.
First look at the paragraph the style of which you like. Then you look at it in code view. Perhaps this paragraph starts with <p class="calibre2">.
Then you look at a paragraph with a different style. In code view you may see that this parapraph starts with <p class="calibre7"> (or any other number). You simply have to change it into calibre2.
As the epub was created with Calibre, you've got the file 'stylesheet.css'. In Sigil you find it in the Stylesheet folder. If you dont need <p class="calibre7"> any more, you can delete this class. In stylesheet.css it is shown as
.calibre7 {
.
.
.
}
George