[FIX] web_editor: fix black and white CSS variables

With [1], the enforced black and white CSS values set on the :root were
not interpolated and rendered as the variable names instead of their
values. This commit fixes that.

[1]: https://github.com/odoo/odoo/commit/e406b084815b19a43e0d92d0f2bf119db230d7f6



closes odoo/odoo#125176

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