wiki-grav/themes/learn2-dark/scss/theme/_header.scss

#header {
    background: $header-bg;
}

.searchbox {
    border: 2px solid lighten($header-bg, 10%);
    background: darken($header-bg, 10%);
    border-radius: 4px;

    label {
        color: rgba(200, 200, 200, 0.8);
    }

    input {
        color: $white;
    }
}