pageButtonTextStyles property does not work

**Bug Description**
When i try to change the color of the buttons from white to any other color, either by setting the `pageButtonTextStyles` property or by setting the `skip- done- back- or nextText` property to a Text Widget with wanted characteristics, they stay white. I'm using intro_views_flutter Version 2.6.0

**To Reproduce**
1. Set the `pageColor` property to `Colors.white`
2. Change the button color to any other than white, using one of the properties above
3. Buttons are still not visible

**Smartphone:**
- Tested on a Samsung Galaxy S7

**Flutter doctor output**
`[✓] Flutter (Channel master, v1.6.2, on Mac OS X 10.13.4 17E199, locale de-DE)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✗] iOS toolchain - develop for iOS devices
✗ Xcode installation is incomplete; a full installation is necessary for iOS development.
Download at: https://developer.apple.com/xcode/download/
Or install Xcode via the App Store.
Once installed, run:
sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
✗ libimobiledevice and ideviceinstaller are not installed. To install with Brew, run:
brew update
brew install --HEAD usbmuxd
brew link usbmuxd
brew install --HEAD libimobiledevice
brew install ideviceinstaller
✗ ios-deploy not installed. To install:
brew install ios-deploy
✗ CocoaPods not installed.
CocoaPods is used to retrieve the iOS platform side's plugin code that responds to your plugin usage on the
Dart side.
Without resolving iOS dependencies with CocoaPods, plugins will not work on iOS.
For more info, see https://flutter.dev/platform-plugins
To install:
brew install cocoapods
pod setup
[✓] Android Studio (version 3.1)
[!] IntelliJ IDEA Ultimate Edition (version 2018.3.5)
✗ Flutter plugin not installed; this adds Flutter specific functionality.
✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] Connected device (1 available)
`