You personally I'd like to thank for the help! I greatly appreciate it.
I just can't love vivaldi for letting me fix what it broke with an update and doesn't allow to change in settings. It is exactly a workaround, not a feature.
For anyone who struggle with the same issue, for ubuntu you can use the last link from mib2berlin comment and this very simple CSS:
* {
font-family: Ubuntu !important;
font-size: 14px !important;
}