Unify outline look for timeline elements

### Steps to reproduce

https://user-images.githubusercontent.com/2803622/177496203-5ec69eb2-c722-44bf-a5cb-49587b99358e.mp4

- message has no outline
- reaction has a pretty contrast rich outline without hovering, and even richer when hovering
- thread summary has no outline by default, and gets an outline that is softer than the reaction's default outline when hovering
- context menu (proper name?) has yet another outline for the whole menu, and no outline for the item that is hovered over


- date border has no outline at all


- composer buttons are totally different yet again (green)
- no outlines (except that chrome accessibility thingy selecting Sticker in the screenshot, which looks different yet)

### Outcome

#### What did you expect?
unified look and feel
#### What happened instead?
see above

### Operating system

arch

### Application version

Element Nightly version: 2022070501 Olm version: 3.2.8

### How did you install the app?

aur

### Homeserver

_No response_

### Will you send logs?

No

Start by reviewing the attached video and screenshots, then locate the timeline UI elements they show: messages, reactions, thread summaries, context menus, date borders, and composer buttons. Compare their existing outline and hover styling, and define a consistent visual treatment across these elements; the issue is complete when the timeline has a unified look and feel.

Written by the indexing model from the issue text.