Categories panel: the active checkbox should be more emphasised

[manfred_a]

The visibility of an activated category is very low, especially on large screens: It is marked by an X inside the box, with a width of one pixel.
Also the box for the parent category (a box with a small triangle at the lower right corner) is very similar to a normal box.

I think colored boxes would be much more better. You already used the grey box if not all files in the selection are assigned to a category.