Issue number: resolves#26596
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## What is the current behavior?
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There is no way to customize the month/year toggle button using CSS.
## What is the new behavior?
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this PR. -->
- Adds a Shadow Part to the month/year toggle button.
## Does this introduce a breaking change?
- [ ] Yes
- [x] No
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and migration path for existing applications below. -->
## 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>
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