These are my Calibre css override settings when I download fic from the A03. :)

body { margin: 0; }
a {text-decoration: none; color: black;}
a [href] {text-decoration: none; color: black;}
p {margin-top: 0pt; margin-bottom: 0pt; padding: 0pt; text-indent: 15pt; text-align: justify;}
h1, h2, h3, h4, h5, h6 { text-align: center; padding-bottom: 10pt;}