Inconsistent look and feel between prometheus and alertmanager web consoles


**What did you do?**
Open prometheus and alertmanager web consoles.

**What did you expect to see?**
Consistent design and theme for both services. They should both have the same look and feel.

**What did you see instead? Under which circumstances?**
The two services have different look and feel. 

**Environment**

* Alertmanager version:

0.17.0

* Prometheus version:

2.10.0
