There is an issue with the Dark (new) theme of the Second Life forums where the header on the [Unread Content] page isn't showing as it should. Apparently it's specific for that theme, the new light one and the classic ones works fine. An example of what I mean is shown in this thread: https://community.secondlife.com/forums/topic/526466-unread-content-header-annoyance/

When using the dark theme, the menu inside

<section data-controller="core.front.streams.main, core.front.core.ignoredComments">

renders unstyled (vertical plain list instead of the styled dropdown).

The light theme loads assets from .../set_resources_10/... , the dark theme loads the same asset paths, but from .../set_resources_11/...

The HTML markup is identical, so the issue is not in the template but in the CSS/JS bundle for set_resources_11. If that bundle is missing files, corrupted, or not rebuilt properly, styling for components like .ipsMenu is lost.

I'm not sure if you host this yourself or as a service, but a possible solutions is to check that all files in uploads/set_resources_11/ exist and then clear caches/rebuild theme resources in the Admin Control Panel to regenerate the bundle.