[Item] add inverted variation

[y0hami]

[Item] add inverted variation

[lubber-de]

Is this really needed? By default all 3 backgrounds for item (item,content,extra) are either none or transparent. So if the body background will be set black, then it works out of the box,

An item is basically only some layout container, rather than a design element.
All possible elements within an item need to be set to inverted separately anyway if desired by the developer.

Of course we can offer inverted item which would just set the background to black by default, but nothing else is needed then IMHO.

Did you have some other behaviour in mind?

[lubber-de]

Ok, i'll prepare a little PR. I just recognized some color values have indeed default (black) settings within item (but still most default values are again empty strings (which is invalid CSS anyway btw, We should set those empty strings to inherit instead?)

[y0hami]

I was thinking if an item was inverted the text for example would be white instead of black.

In this case (below) this is a list of items inside an inverted segment.

[lubber-de]

Fixed in current Release https://github.com/fomantic/Fomantic-UI/tree/2.6.4