Dark Mode support

[karishmapujara]

@viniciussoares
In iOS 13,

If dark mode is enabled in the device, then Date Picker Dialog is not showing properly.

[leonluc-dev]

I have created a dark mode compatible version of the DatePickerDialog here: https://github.com/leonluc-dev/DatePickerDialog-iOS-Swift/blob/1c2c6ef/Sources/DatePickerDialog.swift which you can add to your project.

Hopefully it can be merged into the main repository soon and be part of the cocoapods package. (It's been waiting as pull request #97 for a while)

[roulljdh]

@leonluc-dev thank you for this! I hope the PR gets approved. Works like a charm.