Bug 26706: Fix btn-default styling for better contrast

Bug 26706: Fix btn-default styling for better contrast

This patch makes some updates to the OPAC's CSS in order to make
Bootstrap button styles more consistent. Some CSS has been simplified
and some has been made more specific.

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

View a variety of pages in the OPAC to confirm that button styles look
good. For example:

 - Advanced search.
 - Catalog search results.
 - Bibligraphic details (including sidebar menu links and menus).
   - Subscription details, brief and full
 - "Your summary" page, various tabs.
 - Lists

The more pages tested the better! Be sure to test the hover and active
states of buttons as well.

Signed off: Lisette Scheer <lisetteslatah@gmail.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>