Improve styling for diff hunks

[clwainwright]

This PR changes the background of the created and deleted diff hunks to use the created_background and deleted_background styles, respectively, and it makes the folded indicator for deleted hunks much more visible. Without this, the backgrounds just use a faded version of the foregrounds, which can make it hard to design themes where the gutter marker is sufficiently bright or saturated and the background is sufficiently dark or unsaturated.

Here are some screenshots with a few different themes. Old is on the left, new is on the right.

Andromeda:

Summercamp:

Rosé Pine Dawn:

Release Notes:

• Diff hunks now use the created_background and deleted_background colors from the theme.
• The deleted diff gutter indicator is larger.

[clwainwright]

@danilo-leal curious why you prefer the fade-out over using created_background? Seems like this would be exactly the right place to use it, if anywhere.

(if we stick with fade-out, should probably change the release notes or open a different PR)