Light theme has poor readability about networkmanager

[BornToBeRoot]

The problem is that the library (Mahapps.Metro) has a value (e.g. like Brush.Gray1-10) and this value is set depending on the selected mode dark/light.

If the color of a text is changed for the light design, it's also changed for the dark design.

I implemented an option to add custom designs: https://borntoberoot.net/NETworkManager/docs/settings/appearance#use-custom-themes

This file can be copied & adjusted: https://mahapps.com/docs/themes/thememanager#creating-custom-themes

Here you can set all (like ~10 colors are used) to change the window & text.

PRs are welcome 😄