ENH: Improve styling of extension items

Apply some corner rounding to the borders that show when hovering over
an item in the extension list. This improves consistency with the
overall look of the site.
ENH: Improve styling of download buttons

Tweak the styling of the download and install buttons to more closely
match the other recently tweaked widgets. The changes are subtle but the
buttons are now much less flat, providing an improved and more
consistent look.