[FIX] web_editor: properly compare CSS variables and CSS values

The editor sometimes compare CSS values, in particular in the
selectStyle method where a given style is not applied if it would have
no effect (e.g. applying an inline pink background-color is useless if
the block is naturally pink because of CSS rules). That comparison
failed when it was comparing 'var(--XXX)' and 'YYY' as 'var(--XXX)' was
not processed as the CSS variable 'XXX' to be read.

Related to task-2599770

closes odoo/odoo#75188

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