[FIX] mass_mailing: enhance mass mailing template preview behavior

Currently, if user clicks the mass mailing template (theme) and holds
the click while moving the mouse (as if performing drag and drop), a
weird white highlight appears on the preview of the template.

This commit fixes the behavior by setting appropriate background color
instead of the default one of drop-down item, when the template is
focused.

task-2812312

closes odoo/odoo#89893

X-original-commit: e49f55e6f34486f8425653323544838f1a9fbd38
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/static/src/scss/mass_mailing.ui.scss

@@ -85,6 +85,10 @@ html, body {
             }
         }
 
+        &:focus {
+            background-color: $o-we-sidebar-bg;
+        }
+
         &.selected .o_thumb {
             border: 2px solid $o-brand-odoo;
             background-color: $o-we-sidebar-bg;