When opening the NotifPanel, new messages should be stylized differently than old messages

### Your use case

A kind of continuation of https://github.com/vector-im/element-web/issues/23402 to further ease noticing new messages in the notification panel since the last time you had it open.

#### What would you like to do?

If I open the notification panel while it has new messages which I haven't seen before, it would be useful if those new messages were styled differently than old messages, so you can tell them apart easily.

#### Why would you like to do it?

To more easily notice which messages in the notification panel are new.

#### How would you like to achieve it?

For example, by shading the new messages' background in a different colour.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_