The header styling is CSS as well. It is from MediaWiki:Badges-blog.css. Here is the particular section.
/* Blog post style */
body.ns-500 .WikiaPage .WikiaArticle .mw-content-text {
font-family: Rubik, Helvetica, Arial, sans-serif;
color: #002a32;
font-size: 18px;
font-weight: 300;
line-height: 1.6;
}
.FANDOM-H2-CC,
.ns-500 .WikiaArticle h2 {
border-bottom: 0;
background-image: -webkit-gradient(linear, left top, right top, from(#dfec24), to(#dfec24));
background-image: -webkit-linear-gradient(left, #dfec24, #dfec24);
background-image: -o-linear-gradient(left, #dfec24, #dfec24);
background-image: linear-gradient(90deg, #dfec24, #dfec24);
background-position: 0 0.9em;
background-repeat: repeat-x;
background-size: 1px 4px;
-webkit-box-decoration-break: clone;
box-decoration-break: clone;
color: #002a32;
font-size: 28px;
font-weight: 400;
line-height: 1.2;
padding: 0 6px;
text-transform: uppercase
}
.ns-500 .WikiaArticle h3 {
border-bottom: 0;
font-size: 20px;
font-weight: 400;
padding: 0 6px;
text-transform: uppercase;
}
.ns-500 .WikiaArticle h2 + h3 {
margin-bottom: 15px;
}