UI tweaks

[yshavit]

• date-range picker can now change style
• "open in new window" button doesn't use fancy icon (it's not available on older macs)
• maybe fix the slightly ugly background for the "updated" message?
• Make the spinner centered, instead of having it take the full width. I originally had it full-width because it would turn into a bar for the determinate phase, but it doesn't do that anymore. On older macOS versions, the pie spinner left-aligns within its box, so making it be full-width makes it appear on the left.