Add new page for Custom Themes [advanced] for 5.2 (dev)

[Siedlerchr]

The 5.2 has the feature for addng a custom theme:

Explain that the LIght theme Base.css is the default and Dark.css inherits from Base.css and just overwrites the colors.

https://github.com/JabRef/jabref/blob/master/src/main/java/org/jabref/gui/Base.css
https://github.com/JabRef/jabref/blob/master/src/main/java/org/jabref/gui/Dark.css

CSS reference for javafx: https://openjfx.io/javadoc/15/javafx.graphics/javafx/scene/doc-files/cssref.html

Add some examples