Bug 21157: Improve style of OPAC login modal

Bug 21157: Improve style of OPAC login modal

This patch revises the style of the OPAC's login form modal view. The
goal is simply to make it look nicer.

To test, apply the patch and regenerate the OPAC's CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

In the OPAC, click the "Log in to your account" link at the top of the
page. It should trigger the login modal. While the modal is displayed,
resize the browser width to confirm that the form handles different
widths well.

Sign off if you think this is an improvement.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>