[FIX] website: restore bg color compatibility for s_media_list

[1] removed the associated bg color of media list items as it was now
defined directly on the XML of the snippet. This was however enough to
break the style of old-db snippets. As the s_media_list snippet was just
refactored to need a 001 version, in the same Odoo version where the
mentioned bug was introduced, this commit can just restore the
background color associated for 000 versions.

[1]: https://github.com/odoo/odoo/commit/5953c4df6b9c08519f64a65c1cf5d5558d59f7d4#diff-62db7349afa5503cc8308e815d41db5dL10

closes odoo/odoo#49697

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/snippets/s_media_list/000.scss

@@ -8,6 +8,7 @@
     }
     .s_media_list_body {
         padding: $grid-gutter-width;
+        background-color: gray('white');
     }
     .s_media_list_options {
         @include o-position-absolute(auto,0,0,0);