"Verify this session" shield is difficult to see on dark theme

### Steps to reproduce


(**the shield in the top left** is difficult to see.)

1. Log into an account for the first time.
2. Switch to the dark theme
3. Verify the device (via another device or passphrase)
4. See that the shield in the header of the "Verify this session" modal is dark and hard to see. It should probably be white.

### Outcome

#### What did you expect?

To be able to see the shield.

#### What happened instead?

The shield is low contrast and hard to see.

### Operating system

NixOS Linux

### Application version

Element version: 1.11.24 Olm version: 3.2.12

### How did you install the app?

nixpkgs

### Homeserver

_No response_

### Will you send logs?

No