By default, this is styled in the core of WP in the login.css and colors-fresh.css
You could always copy this into you theme's CSS
input.button-primary, button.button-primary, a.button-primary {
border-color: #298CBA;
font-weight: bold;
color: white;
background: #21759B url(../images/button-grad.png) repeat-x scroll left top;
text-shadow: rgba(0, 0, 0, 0.3) 0 -1px 0;
}
And make the changes you'd like to see and simply add the !important
to the ones that are different. Like
input.button-primary, button.button-primary, a.button-primary {
border-color: #000 !important;
font-weight: normal !important;
color: orange !important;
text-shadow: none !important;
}
As far as changing the 'text' of the button, that's part of the WP core file and would be over written each time you updated and would not be transferred from site to site.
There's a couple of plugins that might help with customizing your login page a little more.
BM Custom Login
Custom Login