The best way to do that would be to edit our template file for the SCLogin module. In the /modules/mod_sclogin/tmpl/joomlaLogin_vertical.php, search for "btn-primary". You should see it on line 47, like below:
<button type="submit" name="Submit" class="btn btn-primary <?php if (!$showRegisterLinkInLogin)
Remove both btn and btn-primary from the class, so it looks like:
<button type="submit" name="Submit" class=" <?php if (!$showRegisterLinkInLogin)
Hopefully, that should get you going, but we'll have to see the results and how the template interacts with the button after the modification to make sure.
Thanks,
Alex