Switch to ID-based style for button.

guacamole/src/main/webapp/index.xhtml

@@ -61,7 +61,7 @@
                         </div>
 
                         <div id="buttons">
-                            <input type="submit" name="login" class="login" id="login" value="Login"/>
+                            <input type="submit" name="login" id="login" value="Login"/>
                         </div>
 
                     </form>

guacamole/src/main/webapp/styles/login.css

@@ -146,7 +146,7 @@ div#login-dialog #buttons {
     text-align: right;
 }
 
-input[type="submit"].login, button.login {
+input[type="submit"]#login, button#login {
 
     background-image: url('../images/guacamole-logo-64.png');
     background-repeat: no-repeat;