keyboard color in themes (suggestion)

[daifkyou]

Is your feature request related to a problem? Please describe.
I'm always frustrated when my eyes hurt.

Describe the enhancement you'd like
Please make the white keys less white in the dark theme. If most everything else in the dark theme is darker, why can't the piano be darker?

[ShinkoNet]

In the next update, the new theme fluent (dark) will have a keyboard that isn't as white.

[Bentroen]

Added in d523dd0.