Thanks @strawberry-3-141 for your hint! It worked perfectly. I edited the custom.css file in the css folder like this:

.dimmed {
  color: #fefefe;
}
.normal {
  color: #fefefe;
}

And also thanks for reading through my little blog. :-)

Thanks for your post!

The brigthness ist on my todo-list. I think i will start working on this issue at the weekend. And of course i can share the CSS here.

Here´s another MagicMirror from Germany: http://www.bussardstrasse131.de/category/magicmirror/.