WIP: Change background colors of dark theme to material default ones

The dark theme colors were set to full black for many screen backgrounds which made the elevation shadows invisible. I removed many of the color overrides so the default gray values for dark themes are used (https://material.io/guidelines/style/color.html#color-themes). This way all main screens (settings, new, swap, etc.) have the same background color and overall color scheme. Also, the app details screen now has much more contrast.

Additionally I removed the green ripple color (which looked a bit off - especially on the blue bottom navigation) so the default gray one is used instead.

Please let me know if I should do more changes or roll back something.