no visible line around the text box

The reason is that your theme’s stylesheet (style.css) applies almost white border to textarea.

https://www.harrykouwenbergaccountant.nl/wp-content/themes/elegant-grunge/style.css
Line #565

textarea {
    border: 1px solid #F2F2F2;
    height: 100px;
    margin: 0;
    padding: 5px;
    width: 410px;
}

Remove or comment out the line if not necessary.