ENH: Improve styling of other appstore filters

Apply similar styling (except the search icon) to the revision filter
field as the search field so that the two are consistent, and also apply
similar styling to the <select> filters (arch, os) for improved
appearance, especially in alternate system color schemes. Also limit the
width of the revision filter field, which both improves appearance
(otherwise the search filter field seems to be off by its lonesome self)
and conserves space.
BUG: Work around combo box styling in Firefox

Add a work-around for styling of combo box (<select>) elements in
Firefox, where 'appearance: none;' is not sufficient to hide the default
dropdown arrow.
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 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.