I don't know if someone has the same issue.
[ Publication 188.8.131.52, version 2021081001 sur Moodle 3.11.2 (Build: 20210729) ]
I try to personalize activities colours.
The "quiz" is set to white (#FFFFFF) however when I check the course it seems it is set to "transparent".
It is the same with all activities or resources (no border colour, and no background colour). This setting is configured.