Fix light/dark mode syntax highlighting

[aykevl]

See: https://www.docsy.dev/docs/adding-content/lookandfeel/#lightdark-color-themes

Previously the code would effectively be in dark mode, no matter whether the website was light or dark mode. This led to situations like here, which I don't think looks very good:

After this change, the syntax highlighting theme changes with the website theme:

[deadprogram]

Thanks for the fix @aykevl now merging.