png backgrounds > lossless compressed & use svgs where supported

[PascalReintjens]

I minimized the close-button-background-images via lossless compression, which makes the file about 4 kB smaller without any visual depreciation :)

Additionally I added svgs for hdpi screens. The linear-gradient at the end of each svg background declaration is only supported by browsers which are able to display svgs too. This basically acts as a fallback so that browsers without svg support will instead show the old png

[CreativeDream]

@PascalReintjens thanks. I will take a look