Improve detail link contrast in dark mode (#8548)

Use our light-mode styling for links in that context.

<img width="627" alt="Screen Shot 2023-11-07 at 4 34 48 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/1e30c3ac-18e2-4663-876c-75c6f8b67d53">

Closes #8519.

docs/stylesheets/extra.css

@@ -36,7 +36,7 @@
   --md-default-fg-color--lighter: var(--white);
   --md-primary-fg-color: var(--space);
   --md-primary-bg-color: var(--white);
-  --md-accent-fg-color: var(--radiate);
+  --md-accent-fg-color: var(--cosmic);
 
   --md-typeset-color: var(--white);
   --md-typeset-a-color: var(--radiate);
@@ -92,3 +92,8 @@
 .md-typeset table:not([class]) {
   display: table;
 }
+
+/* See: https://github.com/astral-sh/ruff/issues/8519 */
+[data-md-color-scheme="astral-dark"] details summary a {
+  color: var(--flare);
+}