Section titles in dark mode are too dark

As shown below, the section titles in dark mode are too dark.
I reproduced this issue with Firefox 127.0.1 and Chrome 126.0.6478.115 on macOS Sonoma 14.5, and Firefox 127.0 on Windows 11 Pro 23H2.


<img width="429" alt="スクリーンショット 2024-06-23 10 43 39" src="https://github.com/common-workflow-language/user_guide/assets/4450141/5a69a6e0-5107-40de-b177-0d6bd0055e33">
