[Calendar] Calendar buttons look the same on non-white backgrounds

[aghArdeshir]

Fixes #2338

What I did

1. Made Calendar buttons (day buttons, disabled day buttons, next/previous month/year buttons) background transparent
2. This is because on white background they looked good, but on non-white backgrounds they started looking uglier
3. I believe this is a better default behavior for a reusable component.
4. However may introduce breaking changes if someone really wanted those white backgrounds on their non-white background.
5. This is how the Calendar looks like after my changes:

(on a white background, it should keep looking as it did previously)