Fix Mark as read button left edge in Origine on mobile (#9441)

At max-width 840px the text button is hidden and only the dropdown toggle
is shown. Origine restores its left border and radius with a rule that
used `#nav_menu_read_all .btn` until #9205 turned the ID into a class.
`.nav_menu_read_all .btn` is now weaker than the generic
`.stick .btn + .dropdown > .btn` and `.group .dropdown:last-child > .btn`
rules, so the toggle lost its left border and rounded left corners.
Use the same selector as the other themes.

Closes #9428

p/themes/Origine/origine.css

@@ -1208,7 +1208,7 @@ button:hover .icon,
 		border-radius: 0;
 	}
 
-	.nav_menu_read_all .btn {
+	.nav_menu .stick .mark-read-menu .dropdown-toggle.btn {
 		border-left: 1px solid var(--border-color-shadow-side);
 		border-radius: 3px;
 	}

p/themes/Origine/origine.rtl.css

@@ -1208,7 +1208,7 @@ button:hover .icon,
 		border-radius: 0;
 	}
 
-	.nav_menu_read_all .btn {
+	.nav_menu .stick .mark-read-menu .dropdown-toggle.btn {
 		border-right: 1px solid var(--border-color-shadow-side);
 		border-radius: 3px;
 	}