System Default Timezone dropdown setting is displayed as greyed out in Administration

STEPS TO REPRODUCE

1. Go to Administer Wiki > Content > Localization
2. On Date / Time section, on Timezone, select "System Default" setting from the dropdown list (if not already selected)
3. Observe the text of the setting and compare it with the other dropdown selections (from above it)

EXPECTED RESULTS

The color of the "System Default" setting is black, like the other dropdown settings.

ACTUAL RESULTS

The  "System Default" setting appears to be greyed out, although it's user editable.

This can cause confusion as it can mislead the user that the dropdown is locked and the time zones cannot be changed.

If a time zone is selected, the text of the chosen option is consistent with the other dropdown settings from above it.