Alertmanager settings text not displayed

### What happened? After upgrading to the most recent version of grafana I cannot view the advanced setting of my Alertmanager instances. I can see the correct payload in the network tab but it is displayed black on black. ### What did you expect to happen? Display the json of the configuration ### Did this work before? Yes it worked in version 11 ### How do we reproduce it? 1. Connect an external alertmanager datasource 2. Try to edit the settings in the alerting section 3. View a black text on black background ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: 12.3.1 OS: Mac Browser: Chrome/Firefox <img width="1859" height="841" alt="Image" src="https://github.com/user-attachments/assets/6f81d355-b615-4e34-b396-2036838077e1" /> ### Grafana platform? Kubernetes ### Datasource(s)? Alertmanager