design-themes/theme_bistro/views/snippets_options.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id="bistro_options" inherit_id="website.snippet_options">
        <xpath expr="//div[@id='so_main_colorpicker']" position="after">
            <div data-selector="section">
                <div class="dropdown-submenu">
                    <a tabindex="-1" href="#" class="dropdown-item"><i class="fa fa-reorder fa-1x"/>Borders</a>
                    <div class="dropdown-menu">
                        <a href="#" class="dropdown-item" data-select-class="">None</a>
                        <a href="#" class="dropdown-item" data-select-class="border-waves">Waves</a>
                        <a href="#" class="dropdown-item" data-select-class="border-lines">Lines</a>
                        <a href="#" class="dropdown-item" data-select-class="border-triangles">Triangles</a>
                    </div>
                </div>
            </div>
        </xpath>
    </template>
</odoo>