So far I checked out a request at https://github.com/Microsoft/vscode/issues/6056 which was closed based on Complete list of theme elements to colorize?. This in turn send to https://code.visualstudio.com/docs/getstarted/themes and https://code.visualstudio.com/docs/getstarted/theme-color-reference but there's no information on scopes.
The closest inspiration source I could found was https://github.com/Microsoft/vscode/blob/master/extensions/theme-defaults/themes/hc_black.json but it's far from a Microsoft issued official complete, systematic list/tree of C# specific scopes.
If it matters, it started from the need to color-differentiate variables by scope i.e. local vs. outer scope, or by access modifier i.e. private protected etc.
Scope names are arbitrary. You can use whatever names you want when writing a language grammar.
Because of that, there is no way to get a complete list of those names.
The official conventions for naming scopes can be found here. This page offers only the most basic guidance, so you'll need to look at some popular themes and grammars and figure out what conventions they use.
EDIT:
If you're interested specifically in C# scope names, you can get them from the corresponding grammar file at C:\Users\<username>\AppData\Local\Programs\Microsoft VS Code\resources\app\extensions\csharp\syntaxes\csharp.tmLanguage.json.