Improve css for userid in realtime widget #20076

Description:

fixes #20072

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

@bx80 adding a ui test would have been too much effort. As we do not yet have separate UI tests for the UserId feature I would have to adjust the OmniFixture or add a complete new set of tests.
In general I guess we would need to add more tests for edge cases, e.g. minimum and maximum length, special chars, XSS values,... Might be something for a new issue maybe.