update css for 404 pages to not look like we are in 1995 and have a cons...

update css for 404 pages to not look like we are in 1999 and have a consistent layout across browsers.

Here is a before/after screenshot:
http://chevrel.org/images/phpbugs/bug63242v2_compare.png

Margins/paddings are explicitely set and no longer depend on browser defaults, better readability of fonts while keeping the typical PHP-blue background color on h1.

I don't think it's possible to add a test for that pull request since it is fixing an esthaetics problem to make look the PHP integrated server more visually appealing for people like me that use it a lot.