[Opserty]

Do you want to change all the submit buttons used throughout the style (e.g. "Post Quick Reply" e.t.c.) or just that one for the login?

If its just that one then you can use this:

<if condition="!$show['member']">
		<!-- login form -->
		<form action="login.php?do=login" method="post" onsubmit="md5hash(vb_login_password, vb_login_md5password, vb_login_md5password_utf, $show[nopasswordempty])">
			<script type="text/javascript" src="clientscript/vbulletin_md5.js?v=$vboptions[simpleversion]"></script>

			<label for="navbar_username">$vbphrase[username]</label>
				<input type="text" class="bginput" style="font-size: 11px" name="vb_login_username" id="navbar_username" size="10" accesskey="u" tabindex="101" value="$vbphrase[username]" onfocus="if (this.value == '$vbphrase[username]') this.value = '';" />
			<label for="navbar_password">$vbphrase[password]</label>
				<input type="password" class="bginput" style="font-size: 11px" name="vb_login_password" id="navbar_password" size="10" tabindex="102" />
			<label for="cb_cookieuser_navbar">
				<input type="checkbox" name="cookieuser" value="1" tabindex="103" id="cb_cookieuser_navbar" accesskey="c" />$vbphrase[remember_me]</label>
				
			<input type="submit" class="cust_button" value="$vbphrase[log_in]" tabindex="104" title="$vbphrase[enter_username_to_login_or_register]" accesskey="s" />
		
			<input type="hidden" name="s" value="$session[sessionhash]" />
			<input type="hidden" name="do" value="login" />		
			<input type="hidden" name="vb_login_md5password" />
			<input type="hidden" name="vb_login_md5password_utf" />
		</form>
		<!-- / login form -->
	</if>

I changed the class of the submit button from "button" to "cust_button". Then go:

1. AdminCP
2. Style Manager
3. Select "Main CSS" from the drop down -
4. Scroll to the bottom
   
5. In either of those boxes you can add the CSS for buttons e.g.:
   
   

   .cust_button {
       border: 1px solid #FFFFFF;
       color: #FFFFFF;
   /* e.t.c. */
   }