bug: IOS dark/light button and item focus does not have sufficient contrast (accessibility issue)

# Bug Report

**Ionic version:**
[x] **4.x**

**Current behavior:**
When tabbing to the IOS dark button, there is no visible change in styles to indicate that the button has focus. (The Material/Andriod dark button has a visible change.)

**Expected behavior:**
A visible change in the styling to indicate it has focus.

**Steps to reproduce:**
1. Visit https://ionicframework.com/docs/api/button, or any 4.x IOS application with a `Dark` (or Angular/etc. equivalent)
2. Tab to the dark button to give it focus