[IMP] portal: review menu's divider design

Set the divider color to match the text color. Decrease the
opacity to half the link one (normally 50%). Adjust horizontal margin,
resize the divider height to match the text.

Part of https://github.com/odoo/odoo/pull/52223
task-2197038

closes odoo/odoo#52223

Related: odoo/design-themes#289
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/portal/static/src/scss/portal.scss

@@ -63,7 +63,9 @@ header {
     ul.nav > li {
         &.divider {
             display: none;
-            border-right: 1px solid $nav-divider-color;
+            margin: $nav-link-padding-y ($nav-link-padding-x * 0.5);
+            border-right: 1px solid;
+            opacity: 0.25;
         }
 
         &.active + .divider {