fix(docs-infra): fix styling of <summary> elements on dark theme (#42620)

Previously, the color of `<summary>` elements was hard-coded to `black`.
This did not work well on the dark theme, where the background color of
the page is also very dark.

This commit fixes it by removing the explicit color style, thus letting
`<summary>` elements inherit the color of their container.

Closes #42616

PR Close #42620

aio/src/styles/2-modules/details/_details-theme.scss

@@ -3,9 +3,5 @@
 @mixin theme($theme) {
   details {
     box-shadow: 0 1px 4px 0 rgba(constants.$black, 0.37);
-
-    > summary {
-      color: constants.$black;
-    }
   }
 }