Create 2 new parameters : overlayStyles and imgStyles

[Banb4n]

• Create overlayStyles / imgStyles parameters because we have to set the zIndex of the overlay and with this new parameters it's really easy to set.
• Remove the background parameter
• We create a constant DEFAULT_OPTIONS for more simplicity

[francoischalifour]

Hi @Banb4n, thank you for working on this. However, changing an API like this is not something we can afford as it'd create breaking changes. This is something we should discuss as an issue beforehand.

For now, if you encounter a problem with the z-index value, you probably first want to check your own CSS styles. If you don't have control over them, you can still override the CSS class .medium-zoom-overlay.

Hope this will already fix your issue. We can decide later on if having styles as options (or any way of style overriding) is necessary as the demand increases.