PURPOSE:
This commit adds a min-height property to event pages so that
the footer does not take the entirety of the screen when a page
has little content.
To not have a strange empty space on the booth event page when no booth
is set or when the event is finished, the related error messages are
now displayed below the cover instead of inside it.
It also removes the default grey background on some templates,
so that the theme background color is used instead.
Task-2831951
closesodoo/odoo#89602
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>