Message list feels too crowded, especially in dark theme. This requires more effort to read messages as necessary, due to them not being easy to get at a glance.

This commit makes the following improvements:

- Message bubble borders are removed in dark theme;
- Text in message bubbles have slightly less horizontal spacing;
- Composer text input has slightly less horizontal spacing;
- "New message" separator becomes "new" with a slightly different style;
- Author names are slightly smaller, their opacity is slightly reduced in dark theme;
- Message datetime are slightly reduced in dark theme;
- Date section `hr` are closer to text, with less spacing;
- Reply-to of a message is slightly smaller;
- "Jump to present" alert is less distracting;
- "is typing" member text below composer is slightly smaller;

This reduces visibility of some information to let message content shine more. The "new" message separator style change prevents having text in the middle, which made middle part of message list too crowded. Date section is not much of a problem because it's stable and present only once a day at most.

<img width="2075" alt="a-before-after-1" src="https://github.com/user-attachments/assets/38b8dd3a-87b7-4fe5-8fd4-606fcc54c422">
<img width="2072" alt="a-before-after-2" src="https://github.com/user-attachments/assets/07345761-4362-4c1d-9375-e9b4891803d3">