Make background of a modal darker, to make it seem more natural

[Philipp000]

Closes #41

As mentioned in the corresponding issue, the background of a modal is currently very bright, and the closing sign is not very visible. I think, it would be better to darken the background, instead of increasing the contrast of the closing sign. Here are two screenshots of the version before and after my change:

Before:

After:

[luis815]

This is great!

[arafel]

Gets my vote. :-)

[Niek]

Any chance to do a new release with this change in it @jloh?