[FIX] website: remove the background color behind the affixed navbar

For some unknown reason (part of a huge refactoring during the BS3 to
BS4 migration at [1]), a background color was applied to the <header>
(behind the navbar) once it becomes fixed (after scroll if the related
header scroll effect has been chosen).
That $light color made no sense as is visible if the header uses rounded
corners but is impossible to edit / remove. As a stable fix, it was
decided to not remove it if the header uses a transparent color, in
which case mixing it with $light actually alters the whole menu color.

Note: this fix is done starting from 14.0 as, in 13.0, there is no
(found) standard possibility to use rounded corners or any other way to
let that $light color appear (except using a transparent menu color, in
which case we want to keep the $light color as a stable fix anyway).

[1]: https://github.com/odoo/odoo/commit/09f40036b9c14161db90c6128861369c91c8c64e



opw-2686695

closes odoo/odoo#89896

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