[FIX] web_editor: restore crop widget backdrop color
[FIX] web_editor: restore crop widget backdrop color

Revert https://github.com/odoo/odoo/commit/14c8b285ef3e11cf8a8387bb211272e6456e4d42
which fixed a scss compilation error but changed the design by mistake.
The 4 digits hexa was not supported in old sass compiler so we use a
rgba() value instead.

closes odoo/odoo#52622

Signed-off-by: Quentin Smetz (qsm) <75b324bb495ac8d4f83496f7c52eda774af7ed7a@odoo.com>

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