pitaya dark: grid color too similar to background color

In pitaya-dark, the background color is `#212946` and the grid color is `#2A3459`. This makes the grid hardly visible. For emphasis, title and axes are also in `grid.color`:

![screenshot](https://user-images.githubusercontent.com/181628/146579359-fede613e-f81a-4bf6-8ae1-7909a254112d.png)
