Add flatlaf support

[BenHerbst]

Skia reto is really good application, but misses a good theme in my opinion.
That is why flatlaf would be a good solution, also to get a dark mode.

[raydac]

very good idea, thanks
I have added support of FlatLaf into snapshot, you can take a look at

[BenHerbst]

Wow, it looks very good.
I use Scia Reto mainly only for mind map and do Plantuml in Doom emacs, but you could maybe also add dark theme / Flatlaf support for the text area. I will replace Minder now with Scia reto. Thank you so much for this awesome mind map app!