Cannot change diff editor to a custom theme based on high contrast

[alexdima]

	monaco.editor.defineTheme('custom-hc-theme-1', {
		base: 'hc-black',
		inherit: true,
		rules: []
	});

	diffEditor.updateOptions({
		theme: 'custom-hc-theme-1'
	});

Observe:

• the two editors don't get the hc-black class name. they get:
  • monaco-editor custom-hc-theme-1 original-in-monaco-diff-editor and
  • monaco-editor custom-hc-theme-1 modified-in-monaco-diff-editor
• the tokens don't switch to the high contrast tokens.