fix(datetime): set default background color correctly (#28809)

Issue number: None
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## What is the current behavior?
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In my work for FW-5822, I found that I had unexpected screenshot diffs.
`--ion-color-light` was not defined in our test infrastructure, and the
white fallback for the datetime background was causing snapshots to have
a white background when they should have had a light one.

## What is the new behavior?
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this PR. -->

- Datetime has the correct background color even when the
`--ion-color-light` token is not defined

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

Introduces style changes if the token is undefined, but it should always
be defined in real-world uses.

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## Other information

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screenshots of how the component looks before and after the change. -->

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Co-authored-by: ionitron <hi@ionicframework.com>