Predefined screen timezone selector visibility is poor in dark mode

[cocarrig]

Describe the bug
When in dark mode on dakboard.com, the time zone selection in the Date/Time section of the Predefined screen has poor visibility due to the typeface color (black).

To Reproduce
Steps to reproduce the behavior:

1. Set account to dark mode in https://dakboard.com/settings/themes
2. Access the https://dakboard.com/options/datetime configuration page, and note that the timezone is hardley visible due to the typeface being black

Expected behavior
Text should likely mimmic other lighter text in platform (#d5d5d5)

Screenshots

Additional context
N/A

[Dan-Peck]

Merged in @ 129af63 and added to the queue for upcoming release to the live site.