Provide access to dark theme

[tobiasdiez]

In #3891 a dark theme was introduced but so far there is no switch in the preference dialog that allows to choose it.

[Siedlerchr]

Refs #382

[OlzClark]

Hi, it's my first time contributing to open source. Mind if I give this a shot?

[Siedlerchr]

Sure, go ahead! Have s look at the contribution guide and get started. Feel free to ask any questions. You can always contact us at our gitter chat

[tobiasdiez]

The relevant classes are: https://github.com/JabRef/jabref/blob/master/src/main/java/org/jabref/gui/util/ThemeLoader.java and https://github.com/JabRef/jabref/blob/master/src/main/java/org/jabref/gui/preftabs/AppearancePrefsTab.java

[Siedlerchr]

We could maybe also take a look at jmetro design: https://www.pixelduke.com/jmetro
to get some ideas

[ngrant123]

Hello,
Is it okay if I work on this? I currently have made an option in the preferences box and the dark theme is implemented correctly. Im about to create a pull request and everything. I might have pushed some code accidentally sorry about that I'm new to open source contributing

[ngrant123]

Yeah I definetly pushed some stuff to the master branch sorry about that you can delete my commit im creating a pull request rn