Bug 21851: Improve style of sidebar forms

Bug 21851: Improve style of sidebar forms

This patch makes minor changes to staff client CSS in order to improve
the style of forms in sidebars. It adjusts the style of nested fieldsets
and gives more room to list items (and the form fields they contain).

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

 - View pages with forms in the left-hand sidebar, especially:
   - Circulation -> Overdues
   - Patrons -> Patrons search results
   - Acquisitions -> Invoices
   - Acquisitions -> Order search results
   - Tools -> Tags

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>