Theme install:
* Move the "based on" string below the rating to avoid wrapping to a second line.
* Reduce the gap between the theme screenshot and description.
* Fix Install button height on mobile.

props obenland, ocean90.
fixes #28148. see #29556.

git-svn-id: https://develop.svn.wordpress.org/trunk@29959 602fd350-edb4-49c9-b593-d223f7449a82