[FIX] web: make disabled date visible in dark theme

Before this commit, calendar view in dark theme did not visually
show the disabled date, such as weekends.

In white theme it's fine because bg-view is white, so gray-200 on
white is visible. In dark theme, however, bg-view is identical to
gray-200, so it's not visible.

This commit fixes the issue specifically in dark theme by showing
disabled dates with gray-300.

closes odoo/odoo#119984

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>