appstore ui fixes

The appstore UI forces style changes in an incomplete manner, which causes anyone with a light on dark theme to have great difficulty reading the text. The system settings should be used to avoid UI color conflicts.
Also the "unknown.png" has a big white square behind it, which makes it hard to read the light colored text when it overlaps the image. I removed the white background and replaced it with transparency. ideally SVG's would be used, because they can easily scale with the window size and they can also be tracked cleanly in RCS :P