[core,popover2] Fix heading colors in tooltips in dark mode

[tomshen]

Fixes #3773

Checklist

• Includes tests N/A
• Update documentation N/A

Changes proposed in this pull request:

• Uses text color to set heading color explicitly to override default header colors in dark mode tooltips
• Fixed in both soon-to-be-deprecated Tooltip and Tooltip2

Reviewers should focus on:

• Does this lead to any unexpected styling issues, since this changes the popover appearance mixin?
• Should I add a "test" example for this case? i.e. adding an example to the tooltip docs that uses an header

Here is a Tooltip2 repro: https://codesandbox.io/s/blueprint-sandbox-forked-u378j

Screenshot

Before:

After:

[blueprint-bot]

Fix heading colors in tooltips in dark mode

Previews: documentation | landing | table

[adidahiya]

seems fine to add here. the alternative would be to put this style in _tooltip.scss / _tooltip2.scss, but I think it makes sense to leave it here as well.

just added some code comments to make the code more clear.

[blueprint-bot]

Apply suggestions from code review

Previews: documentation | landing | table