Improve support for Windows high-contrast mode

[terrill]

Improve support for Windows high-contrast mode

[terrill]

Resolved the border problem in CSS by removing borders from controller inputs and replacing them with outline properties on focus. Fixed in 78e6d1d

Resolved the black vs white PNG problem by checking for the user's computed background-color and converting the RGB value to grayscale. If it's < 128, it's a dark color, so use the white buttons. Otherwise use the black buttons. Fixed in a472c87

[terrill]

I'm reopening this issue for at least a couple of reasons:

• Some high contrast modes make the borders around buttons prominent, which from my perspective result in a cluttered interface. Need to test this with high contrast users.
• The volume slider is rendered invisible in some high contrast modes.

[terrill]

High contrast mode has been much-improved (tested using all Windows high contrast modes; Firefox color preferences; and Chrome using Google High Contrast Extension). Specifically:

• Replaced all <button> elements with <div role="button"> on the controller. See #400 for more about this.
• Removed outlines from the controller buttons.
• Removed unnecessary borders and outlines from the overall design as these create a lot of clutter when presented in a high contrast foreground color, and their absence is barely noticeable in default contrast.
• Synchronized the display styles of the volume slider and seekbar (the former was invisible in some high contrast modes; the latter was not).