fix(datePicker): ensure one is able to override default sass variables values

[sryzycki]

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[sryzycki]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[ThomasBurleson]

@jelbourn - I assume you had specific reasons why you used !default... so their removal would be problematic ?

[jelbourn]

LGTM. I just forgot to add !default when I wrote them.