One way to test this on Zorin OS is to try using a third-party theme and see if that shows the highlight. If it does... Then it narrows it down to being likely the Zorin Theme in combination with Vivaldi.
I have Vivaldi installed for testing purposes and have it open, now.
I will need to Undo My Custom theme in Vivaldi to test this against the Zorin Themes, so this may take a few minutes...
But...
We also could give you that custom theme, as we did with Frog, in order to solve your issue.
EDIT: Yes, just tested Vivaldi using default config with Zorin-Grey-Dark
Cannot see highlighted text.
Switched my gtk theme to Azenis2021 and then was able to see highlighted text in a Blue Highlight color.
In This Case the above tip in post number 2 really should have worked. Did you ensure the file was saved properly, with the correct placement and name? Did you restart Vivaldi browser?