selectionBackground has no effect

[LaKing]

Hi, ..

I'm using the cobalt theme from thememirror. The only problem I face with this theme, is that the selected text highlighting needs more contrast, it is hard to see the selected area.

I've tried a few approaches, but couldn't make it.
It seems like selectionBackground has no effect.
Pure css seems to be overridden, at least for the background.

What is the recommended method for setting the highlighted text?

[LaKing]

For googlers finding this, the workaround is

let highlightTheme = EditorView.theme({
  "&.cm-focused .cm-selectionBackground, ::selection": {
    backgroundColor: "#D00"
  },
}, {dark: true});
    
extensions.push(highlightTheme);

[JinXJinX]

There is a typo in the create-theme.ts.
This PR fixed that #7