Where can i find css code for Admin Login page?

viki,

What are you trying to do exactly because it might be a better idea to mod your functions file instead.

First of all i want to replace this image which is showing when we go to admin login. This is the image https://www.example.com/MyBlog/wp-admin/images/logo-login.gif

One thing i can do directly replace this image with the new one but i want to edit the css code for this because my new image has different dimensions.

Viki, put this in your functions.php and modify according to your needs

function login_css_mod() {
$style = '<style type="text/css">
h1 a {
    background: url("https://mysite.com/images/logo.png") no-repeat center top;
    display: block;
    height: 133px;
    overflow: hidden;
    padding-bottom: 15px;
    text-indent: -9999px;
    width: 326px;
}
</style>';
echo $style;
}
add_action( 'login_head', 'login_css_mod' );