In dark mode, Ink.Button now uses the accent color by default, instead

of a grey. This helps it stand out and fit in with other controls.

There's no visual distinction between primary and standard buttons in

this case at this time on dark mode.
On light mode, buttons appear the same as before.
The accent color for both primary and dark mode buttons can be

controlled by setting new CSS variables:

--rb-c-button-accented-bg
--rb-c-button-accented-active-bg
--rb-c-button-accented-hover-bg
--rb-c-button-accented-text-color

Made use of this in Review Board.
Verified appearance in Storybook.