I have (on an idle day) been experimenting with the Dark Them that Death provided in 2021 for the V4 Final. It fixes some frustrations I have found with the previous Custom UI I was using.
It works well for me on the 1 Dec 2023 beta, but I have hit a small bug. If I close MoI when the browser window is closed, on restart I get an error box which says

"TypeError: null is not an object (evaluating 'items[i].expanded = true')

DeathPane.js line 20

18: var items = moi.ui.sceneBrowser.rootItems;
19: for ( var i = 0; i < items.length; ++i )
20:>> items[i].expanded = true;"

If the Browser window is open when I close MOI it doesn't happen.

It's not a problem, clicking OK to the error opens MOI without a browser window.

This must be a startup script.
Back up the script and change it to this:

if (moi.ui.showBrowserPane) {
var items = moi.ui.sceneBrowser.rootItems;
for (var i = 0; i < items.length; ++i)
items[i].expanded = true;
}

Thanks, that fixed it.