They exist but don't actually grab system colors (to prevent pages spoofing the system look).

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Shameless plug:

In neutrino (my webview wrapper), we grab those and inject them as prefixed CSS variables (--neutrino-NameOfSystemColor).

Works on windows/macos/gtk/qt, and except for qt it works live (if the user changes the accent tone, the CSS variable updates automatically).

https://alganet.dev/neutrino/ Screenshot of Mint: https://imgur.com/a/WyjpwsR