Fix a bug on iOS 18 where the timeline background would use the wrong colour scheme when using the app switcher.

[pixlwave]

Use the UIColors directly instead of wrapping the SwiftUI Colors.

Before	After

[github-actions]

	Warnings
⚠️	Some of the commits are missing ticket numbers. Please consider squashing all commits that don't have a tracking number.
⚠️	You seem to have made changes to views. Please consider adding screenshots.

Generated by 🚫 Danger Swift against 80685b7

[stefanceriu]

Suuuper nice 👌

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
824	2	822	0

View the top 2 failed tests by shortest run time

PreviewTests test_joinRoomScreen()

Stack Traces | 9.42s run time

failed - Snapshot "Unknown" does not match reference.

@−
"file:.../__Snapshots__/PreviewTests/test_joinRoomScreen-iPhone-16-pseudo.Unknown.png"
@+
"file:.../tmp/PreviewTests/test_joinRoomScreen-iPhone-16-pseudo.Unknown.png"

To configure output for a custom diff tool, use 'withSnapshotTesting'. For example:

    withSnapshotTesting(diffTool: .ksdiff) {
      // ...
    }

The percentage of pixels that match 0.9593811 is less than required 1.0
The lowest perceptual color precision 0.39937502 is less than required 0.98 (.../PreviewTests/Sources/PreviewTests.swift:66)

PreviewTests test_timelineItemBubbledStylerView()

Stack Traces | 16.5s run time

failed - Snapshot "Pinned messages" does not match reference.

@−
"file:.../__Snapshots__/PreviewTests/test_timelineItemBubbledStylerView-iPad-en-GB.Pinned-messages.png"
@+
"file:.../tmp/PreviewTests/test_timelineItemBubbledStylerView-iPad-en-GB.Pinned-messages.png"

To configure output for a custom diff tool, use 'withSnapshotTesting'. For example:

    withSnapshotTesting(diffTool: .ksdiff) {
      // ...
    }

The percentage of pixels that match 0.99940014 is less than required 1.0
The lowest perceptual color precision 0.7782813 is less than required 0.98 (.../PreviewTests/Sources/PreviewTests.swift:66)

To view individual test run time comparison to the main branch, go to the Test Analytics Dashboard

[pixlwave]

Same on the tests as #3323, they're being flakey on CI.