suggestion for NEWS.md

[tyner]

In NEWS.md we have:

* The color scheme used in the default graphical settings has been
  updated to use modern palettes, thanks to Achim Zeileis. It is also
  easier to customize the color palette in `standard.theme()`, via a
  new function `custom_theme()`.

I'm wondering if it would be helpful to include a hint there as to how users can recover the previous default theme?

[deepayan]

Done.