twitter/bootstrap — pull request #5124 by neersighted (2012-09-13 18:15:18+00:00)
Title: Fix horizontal centering of `.alert .close`

Fix the horizontal centering of `.alert .close` by adding
an extra 5px of `line-height`.

Because of Bootstrap's new default `line-height` of 20px, the `.close`
button was no longer horizontally centered. #492 and #967 both fixed
this issue, but later were regressed. This commit fixes that.