Suggestions for the time slider

[vquach2404]

Samples team has added a new sample featuring the time slider. A few suggestions came up during the PR process that we'd like to share:

• implement with init?(coder:) so the time slider can be used in the storyboard. See Phil's comment for more info
• support dark mode. See Ting's comment for more info

[mhdostal]

@vquach2404 Thank you for the issue! Those are both useful enhancements.