[FIX] website: theme color palette based on primary or alpha

Steps to reproduce:
- install website
- install the theme "clean"
- go to website > customize > customize theme
- select the first "extra color" (alpha) and change it

Previous behavior:
All extra color that have not been set previously have changed
automatically without notice

Current behavior:
the theme is either showing primary/secondary colors or only "extra" colors
Good themes are supposed to enforce primary = alpha and secondary = beta
All extra color that have not been set previously change automatically but
an information tooltip shows up when you hover alpha to avoid confusion

opw-2180611

Forward-Port-Of: #46099
closes #49730

closes odoo/odoo#49789

X-original-commit: c9aada53b1e6b24da4e1fe9ac4d44db3ff8cacda
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>