I added a "workaround" to not style build-monitor pages at all (https://github.com/jenkinsci/simple-theme-plugin/commit/cc68d1a82909f6e1fa1f9b4ec2a85b05d738383b) since all styles on GitHub are not aware of that plugin and therefore break its design.

Unfortunatly some users want to style the build-monitor and now aren't able anymore. This should therefore be user-configurable. Maybe tackle support for styling BlueOcean in a similar way (opt-in, see JENKINS-49081)