Improve readability with better contrast

[peastman]

When using the light theme, the forecast is displayed in black text on a dark blue background. This is hard to read. Using a lighter background (preferably white) would provide more contrast and be easier to read.

The widget has similar issues. It uses white text on a partly transparent background. Depending on your wallpaper image, this can be hard to read. It would be better to use an opaque contrasting background, preferably black text on a white background for light theme and white text on a black background for dark theme.