A few links:
For the latest variables (sometimes the above docs are a bit behind), their default values, and the rest of the css, I recommend grabbing a copy of the app.css and saving it somewhere for reference. You won’t edit this as it won’t do anything; it’s just a quick reference for if you don’t want to use that section of the dev tools.
Open the DevTools “Sources” tab →
Click on app.css →
Select all of this and paste it into a file somewhere.
Also, most of the custom variables in the default theme are declared on body, not :root, so I’d go with
body {
--background-primary: gold;
}
or
.theme-light {
--background-primary: gold;
}
in this case.