Small default CSS improvements

[windymilla]

1. The first line of a centered paragraph is offset by (half?) the text-indent if the user has changed the default paragraph CSS to add a text-indent. This causes misalignment of centered text. Fix by forcing text-indent to zero for centered paragraphs, either directly on the "center" class, or for "p.center".
2. Replace the 0.51/0.49 paragraph margins (related to ancient browsers from the CSS2 era) with 0.5/0.5