[FIX] web: `daterangepicker`, website conflict

The 'transparent' background value has been added by https://github.com/odoo/odoo/pull/99755

 to
correctly handle color variations for the backed.
The rule was inadvertently overriding custom designs defined by
website_sale_renting (frontend).

This commit allows to inject dark-mode customization using CSS variables
, while keeping a fallback value used by the frontend.

task-3015397

closes odoo/odoo#103061

Related: odoo/enterprise#32705
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web/static/src/legacy/scss/daterangepicker.scss

@@ -98,7 +98,7 @@
             }
 
             &.off {
-                background-color: transparent;
+                background-color: var(--daterangepicker__cell-bg--off, #{rgba($secondary, .1)});
             }
 
         }