Improvements to dark mode and news

Add dark mode to splash screen

In previous versions the dark mode experience felt like this:

@Moizer02 suggested making the splash bg color match the time, which is implemented since this version. If you don't notice the change restart GVM again after updating to this version.

Minor improvement to news section

• " (") was added to the list of handled html entities
• Better validation of the news data and handling of errors
• and a little eye candy. Smooth transitions between placeholder images and downloaded ones in news thumbs and avatars