Fix dark mode detection for system theme for highlight unfolded comments

Author: cherryblossom000

Highlight-Unfolded-Comments/StackHighlightUnfoldedComments.user.js

@@ -57,7 +57,9 @@ const observeContainer = (commentsContainer) => {
         }
         for (const comment of newComments) {
             window.setTimeout(() => {
-                const highlightColor = document.body.matches('.theme-dark')
+                const highlightColor =
+                  document.body.matches('.theme-system') && window.matchMedia('(prefers-color-scheme: dark)').matches ||
+                  document.body.matches('.theme-dark')
                     ? '#403d33' // Dark brown, close to default dark background
                     : '#fff2e0'; // Pale yellow, close to default light background
                 // eslint-disable-next-line no-param-reassign