I think that the problem is in the file /usr/share/themes/Breeze/gtk-3.20/gtk3.css and gtk-dark.css, or the theme you use.

  .solid-csd decoration {
    border-radius: 0;
    margin: 0;
    padding: 0px;
    border: none;
    background-color: #556068;
    box-shadow: none; }

If I change “margin: 2; , Remmina and Tilix have borders again,