cmd/present: Fix indentation in styles.css

No functional changes.

Also adds a newline to the end of the file, for compatibility with
`cat(1)` and `vim(1)`. See also:

https://gcc.gnu.org/ml/gcc/2003-11/msg01568.html

Change-Id: I4fc7ac64ad7c4b4d9c2be7296cb8b612e0becef2
Reviewed-on: https://go-review.googlesource.com/2106
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>