New version of Jenkins (2.226) is available for download (changelog). [Manage Jenkins]
This exists exclusively in the popup, and not on Manage Jenkins which isn't really clear from your report (inferred from "Manage Jenkins" link).
.page-header__hyperlinks A has a margin-right: 0.25rem; from base-styles-v2.css.
Pinging Félix Queiruga Balado – seems like the UI refresh caught more elements than expected due to the popup.
Steps to reproduce:
docker run -p 8080:8080 -p 50000:50000 jenkins/jenkins:2.225 and get through the setup.
The issue comes from the fact that the HTML code for the monitors popup is located within the header and the .page-headerhyperlinks div. I already needed to make a hack so that the header styles would not apply here (https://github.com/jenkinsci/jenkins/blob/144550b2fc4ef59ce1af0abe4dace9e4fe394c89/core/src/main/resources/jenkins/management/AdministrativeMonitorsDecorator/resources.css#L92). A solution would come from resetting the margin there as well.