Added getStyleSheets Fucntion

[JellevanAbbema]

Adds the functionality to change the style of the preference window
This function already exists in the Java 8 version, this pull request adds the exact same code for Java 11.

PR Checklist

• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open an issue first and wait for approval before working on it.
• The code follows the Google Java Style Guide.
• JavaDoc is added / changed for public methods.
• An example of the new feature is added to the demos.
• Documentation of the feature is included in the README.
• Tests for the changes are included.

What is the current behavior?

Unable to change stylesheet in Java 11 version

What is the new behavior?

Able to change stylesheet in Java 11 version

[JellevanAbbema]

So you guys had a random dot in a pom file which broke the entire build, removed it in this commit.
I have no idea why the builds here online are failing but from what I can tell it's not my fault.

[dlemmermann]

The build fails because it uses a third party code analysis tool for which only I have the permission to run it. Stupid .... I need to change that.