@Miraak49 Oh yeah I think you're right, this changed a long while back, some Chromium/Blink changes.
No idea what to do, but it's not Vivaldi UI so can't be changed with UI CSS, and there's no setting for it. Could maybe be done with some user-script or a user-style extension like Stylus.
Google it I guess? 😉
Also, shows that it might be an idea to upgrade your browser more often, you'd pick up the change when it happens instead of relying on people remembering whatever minor cosmetic change that happened over a year ago...