docs(datepicker): custom datepicker header demo styling not working

[crisbeto]

Fixes some styles in the datepicker header example not working due to style encapsulation. Also takes the chance to make the text less bold.

For reference:

[mmalerba]

Will this change anything about the encapsulation? It seems like we just moved it from a separate file to string literals.

[crisbeto]

It does. Previously the styles were encapsulated to the demo component, but now they're moved to the custom header. I moved them into the literal, because we have the template in a literal as well, which makes the example a little more concise.

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