Dark mode: highlighter background too light for text

In dark mode, the Text Highlighter Demo component renders light-colored text on a light-colored highlight background.
This results in very low contrast, making the text hard to read.

affected component: highlighter-demo

problem screenshot: 

<img width="1010" height="693" alt="Image" src="https://github.com/user-attachments/assets/a0673228-246b-4703-8fbf-a2f7d5f64fca" />