ENH: Improve styling of extension buttons
ENH: Improve styling of extension buttons

Tweak the styling of the download and [un]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.

midasplatform/slicerappstore,midasplatform/slicerappstore,midasplatform/slicerappstore