Bug 32022: Style tweaks to fieldsets and page-section

Bug 32022: Style tweaks to fieldsets and page-section

This patch makes CSS changes to try to bring some consistency to
various elements which are similar in the general sense of "white box."
Fieldsets, page-section, etc.

To test apply the patch and rebuild the staff interface CSS.

Test various pages with page-section, fieldsets, etc.:

- Patron details
- Patron edit
- Patron accounting
- Check Tools -> Patron lists -> New to confirm that there is not a gap
  between the submit button's container and the rest of the form.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>