docs/Writerside/cfg/static/ucx.css

html.theme-light {
    --ucx-color-accent: #5b6f7a;
}
html.theme-dark {
    --ucx-color-accent: #215c59;
}

.article__h1 {
    margin: 0;
    padding: 0;
    .title {
        display: block;
    }
    .title__content {
        display: block;
        color: white;
        background-color: var(--ucx-color-accent);
        padding: 0.1em;
    }
}

.article__h2 .title__content {
    border-bottom-color: var(--ucx-color-accent);
    border-bottom-style: solid;
    border-bottom-width: 1pt;
    width: 100%;
}