Finalize design for Toolbar warning message, and change the install screen design to match

Problem/Motivation

This is a follow-up to #2938800: Finalize wording for Toolbar warning message to finalize the design of the toolbar warning message provided by the Umami Demo install profile (since that issue wound up only being about the wording in the end).

The scope of this issue is to figure out what the warning message should look like - colors/contrast/icons/etc.

Current state (following #2938800: Finalize wording for Toolbar warning message):

In addition, since the message you see when choosing the Umami profile during Drupal installation is similar to the above, its design should be similar as well. So if there are any major changes made in this issue (e.g. removing the warning icon) there should probably be corresponding changes made to the install screen.

Current state of the install screen:
TODO: Add screenshot following #2938803: Remove the warning about the Umami profile on the install screen

Note: This issue is tagged as an "Umami stable blocker" based on https://www.drupal.org/project/drupal/issues/2938800#comment-12465960 and subsequent comments. Fundamentally, it is OK to have warning icons all over the place for something that is experimental, but once it becomes stable, it should be designed in a way that doesn't shove warning messages in people's faces so often :)

Proposed resolution

To define

Remaining tasks

• Define the visual changes needed to tone down the warning (see #2938800-3: Finalize wording for Toolbar warning message and links therein). One possibility, as suggested in that comment, would be to replace the warning icon with an informational icon.
• Define other visual changes (see #2938800-4: Finalize wording for Toolbar warning message for some other suggestions, including color changes, how the link is handled, etc).
• Accessibility review

User interface changes

To define