summary.outline buttons: arrow unreadable for .contrast

[elenes]

Describe the issue

Current Behavior

An accordion like

<details>
  <summary role="button" class="outline contrast"></summary>
</details>

will render the arrow in the wrong color because this css rule

[data-theme=dark] details summary.contrast[role=button]::after {
  filter: brightness(100) invert(0);
}

does not account for .outline

Expected Behavior

The arrow should render correctly (un-inverted for darkmode, inverted for light mode).

It may make sense to simply use --pico-[color] for all outline arrows. This will also make the appearance of the arrows consistent with the border color.