Hi Bianca, to change the background transparency, there is the field Opacity in the Overlay section on Settings > Media. Give it a value 1 to completely remove any transparency.
Changing the close button appearance will be more difficult. We have an experimental plugin available that replaces the standard close button but it will look different to what you want. You’d have to create your own alternative button image to replace the one in that plugin.
Or we could try to construct some custom CSS to get the close button to look like you want…
Let me know what you prefer 🙂