Set white background for inputs and multiple selects

This is for desktop environments/browsers which default inputs and selects to colours other than white. For example, my current one defaults to a light grey, which doesn't work that well with the rest of Symphony's UI.

I've deliberately not done the same for selects which are not multiple as doing so has negative consequences on their borders.

@nilshoerrmann, could you check this doesn't do anything bad in browsers in Mac OS, please?