fix(material/datepicker): improve range selection contrast in comparison range example

[shubhamavhad96]

Context

This pull request improves the visual contrast of the comparison range selection in the Material Datepicker demo. The goal is to enhance accessibility, ensuring the component looks good and is easy to use in both light and dark themes.

Changes Made

Adjusted the background color styles in the date range example to improve contrast

Verified that the new colors meet WCAG accessibility contrast requirements

Testing Done

Manually tested the changes in light mode

Manually tested the changes in dark mode

Confirmed that arrow selection and hover behavior still works as expected

Notes

No screenshots were added because the change is subtle and was visually confirmed to work well in both themes.

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[shubhamavhad96]

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[shubhamavhad96]

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[shubhamavhad96]

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).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

@googlebot I signed it!