[FIX] web_editor: prevent overriding of toolbar CSS by themes

While waiting for a larger fix of the themes, force prevent overriding
of the web editor's UI CSS by themes. This is achieved by applying
`!important` on all web editor UI CSS. This is a temporary fix until
the themes CSS is fixed and/or the new web editor library is ready.

closes odoo/odoo#31721

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