Gallery improvements

[sergejbohlender]

• [x] opacity of download button
• [x] background image, so its not so empty
• [x] center gallery
• [x] remove wird gallery spacings
• [ ] non transparent download text

[sergejbohlender]

@danblamar have a look on the intermediate state

[danblamar]

Button while cursor's on looks fine ✅ Button while cursor's off, would look better if only the background looked transparent, but text remain fully opaque.

It'd be nice to give a try ;)