Fix css centering on mobile

[gusano]

On some mobiles the modal is not centered (see screenshots below).
This fixes it by using flexbox.

Before

After

[gusano]

I don't know how minifying css is done (didn't use Grunt in a looong time) but it looks like dist/ekko-lightbox.min.css hasn't changed since a year ago, otherwise I would happily add the change there as well.
Meanwhile I'm gonna use my fork :)

Thanks for the nice lib!

[ashleydw]

As this plugin uses bootstraps modals, isn't this a problem in bootstrap rather than the plugin?

[VirginieGarcin]

Indeed it's the default behaviour of bootstrap modal.
It's not only on mobile: bootstrap modal is always shown at the top of the page. (On desktop as the display is landscape we will not notice it for classic pictures, except for panoramic images.)

Even if it's the default modal behaviour, the result would be nicer if the image was centred vertically :)

[RafaPolit]

There is an un-closed bracket on the CSS file, probably on line 12.

If you meant to include everything inside the container, then the indentation and bracketing is still wrong.