[FIX] mail: starred counter should have muted background

Before this commit, all badge color matched the messaging menu
color badge. Most badge counter in discuss show needaction counter,
so it makes sense to have a catchy color scheme (e.g. white text on
red background).

For starred counter in discuss sidebar, the counter is meant to see
at a glance how many messages are starred, without it being too
catchy.

This commit fixes the issue by introducing a `o-muted` modifier on
`o-discuss-badge`, which displays a grey background for starred
counter.

Also fixes a bug in dark mode where badge background color was
incorrect.

closes odoo/odoo#130452

X-original-commit: de232c330140fd4156e7f416fddfce88b21e71c5
Signed-off-by: Didier Debondt (did) <did@odoo.com>
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/core/common/core.dark.scss

@@ -1,3 +1,3 @@
 .o-discuss-badge {
-    background-color: $primary;
+    --o-discuss-badge-bg: #{$primary}; // sync with --o-navbar-badge-bg
 }

addons/mail/static/src/core/common/core.scss

@@ -10,6 +10,10 @@
     --o-discuss-badge-bg: #{$o-success}; // sync with --o-navbar-badge-bg
     color: $white !important;
     background-color: var(--o-discuss-badge-bg) !important;
+
+    &.o-muted {
+        --o-discuss-badge-bg: #{$gray-400};
+    }
 }
 
 .o-discuss-badge, .o-discuss-badgeShape {

addons/mail/static/src/core/web/discuss_sidebar_mailboxes.xml

@@ -30,7 +30,7 @@
             <span
                 t-if="mailbox.counter > 0"
                 class="o-mail-DiscussSidebar-badge o-discuss-badge badge rounded-pill ms-1 me-3 fw-bold"
-                t-att-class="{ 'bg-400 text-light': mailbox.id === 'starred' }"
+                t-att-class="{ 'o-muted': mailbox.id === 'starred' }"
                 t-esc="mailbox.counter"
             />
         </button>