Fix ActionList.Item hover and focus background color

[colebemis]

ActionList.Item is missing a hover and focus background color in dark high contrast and color blind themes.

This makes hover and focus states invisible in new Projects dropdown menus when viewing in dark high contrast or color blind themes.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://primer.style/react/storybook?path=/story/composite-components-actionlist--single-select-list-story&globals=dayScheme:dark_high_contrast
2. Notice that hovering or clicking on items in the list doesn't change the background color

Expected behavior

ActionList.Item should change background colors when hovering or focusing. See example in the dark theme:

Additional context

The hover and focus background colors are currently hard-coded in the ActionList.Item component (source) because it was built before the new functional color system variables where published. Now that the new functional variables are published, let's use those instead. Whoever takes this on should work with @auareyou to pick the right color variables.

[tallys]

update: Working session with @colebemis @mperrotti and @auareyou on the books for next week - looking to use the functional variables to unblock this work and scope longer term considerations around states and themes.

FYI @alliethu (getting this on your radar bc this can affect accessibility)

[auareyou]

As discussed in the work session with @mperrotti and @colebemis, @Juliusschaeper and I added the provided hardcoded values to Primitives and proposed a solution for the high contrast hover state. This work is being done in this PR.

[vdepizzol]

The work in primer/primitives seems to be ready at primer/primitives#256.

@Juliusschaeper has documented his high contrast explorations in this Figma file. The high contrast values seem to be following the exploration¹, although there's still an open question over how the borders can be implemented in high contrast.

Footnotes

1. with a gray selected state instead of a blue one, to make a distinction from focused states ↩

[broccolini]

Status update from planning meeting: WIP primer/primitives#259 - @mperrotti is going to review with @vdepizzol this week.