Helllo
where can I edit Post reply Button style
no I mean the buttons like "Post Reply, Preview post, Search Thread, Go"
Those use the input element, so you can easily style them in your CSS. Go to Admin CP > Templates & Style > Themes > Your Theme > global.css > Advanced Mode, put this in the bottom and change it to your likings:
input.button {
background #000;
color: #fff;
}