src/vs/editor/browser/services/abstractCodeEditorService.ts/DecorationCSSRules/resolveValue
class DecorationCSSRules:	private resolveValue(value: string | ThemeColor): string {
		if (isThemeColor(value)) {
			this._usesThemeColors = true;
			const color = this._theme.getColor(value.id);
			if (color) {
				return color.toString();
			}
			return 'transparent';
		}
		return value;
	}