Sorry if my thread seems stupid but on this 3.5.3, I have changed the H3 in the CSS.php (even in the oldcss.php!) with the editor but nothing happens ! Have I done something wrong ?
Thanks a lot

advice you to use the amazing firebug extension for firefox (getfirebug.com)
it lets you check if your css recognized in the first place, and if it gets overridden by other css (and which other css) as well
for the rest, hard to answer without a link or knowing what you did

use the theme option CSS Inserts to override any CSS. That way you won't have to do it all over after an upgrade.