### Preflight Checklist

* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  * v11.0.3
* **Operating System:**
  * macOS 11.0.1 (Big Sur)

### Expected Behavior
Values returned from `systemPreferences.getAccentColor()` should match actual system accent colors.

### Actual Behavior
Values returned by Electron are slightly off (too dark). 

Here's a comparison of 1) values returned from a native app querying the value of `controlAccentColor` on Big Sur, and 2) `systemPreferences.getAccentColor()`, which also queries controlAccentColor. Color column denotes the built-in accent color options in macOS (in `System Preferences > General > Accent color`).

| Color  | Native  | Electron |
| ------------- | ------------- |  ------------- |
| Blue  | #007AFFFF  | #0A5FFFFF
| Purple  | #A550A7FF  | #923796FF
| Pink  | #F74F9EFF  | #F2318DFF
| Red  | #FF5257FF  | #FC3845FF
| Orange  | #F7821BFF  | #F36D16FF
| Yellow  | #FFC600FF  | #FEBC09FF
| Green  | #62BA46FF  | #53B036FF
| Graphite  | #8C8C8CFF  | #797979FF

The values returned from `getAccentColor` are just a bit too dark. And if they're not accurate, they're not very useful, since the point of using `getAccentColor` is to create UI elements that match the user's OS exactly.

### To Reproduce
Here's a [gist](https://gist.github.com/jcarpenter/4c90645e0c4b0035bbdf6f0112d653c0) with a bit of swift for outputting the value of `NSColor.controlAccentColor`.

### Screenshots

Here's what the delta looks like:

![electron-accentColors-test](https://user-images.githubusercontent.com/1425497/102383803-cea1b500-3f80-11eb-92c4-7142313b082a.png)

### Additional Information
The problem might lie in how the value is being [converted and returned](https://github.com/electron/electron/blob/master/shell/browser/api/electron_api_system_preferences_mac.mm#L391): `return base::SysNSStringToUTF8([sysColor RGBAValue]);`
🤷‍♀️
shell/common/color_util.cc
if (color_string.empty() || color_string[0] != '#')
    return SK_ColorWHITE;
  
  std::string source = color_string.substr(1);
  if (source.size() == 3)
    source.insert(0, "F");
  else if (source.size() == 6)
    source.insert(0, "FF");
  
  std::string formatted_color;
  if (source.size() == 4) {
    for (size_t i = 0; i < 4; ++i) {
      formatted_color += source[i];
      formatted_color += source[i];
    }
  } else if (source.size() == 8) {
    formatted_color = source;
  } else {
    return SK_ColorWHITE;
  }
  
  
  std::vector<uint8_t> bytes;
  if (!base::HexStringToBytes(formatted_color, &bytes))
    return SK_ColorWHITE;
  return SkColorSetARGB(bytes[0], bytes[1], bytes[2], bytes[3]);
}
std::string ToRGBHex(SkColor color) {