Right now configured headers are only set on requests made from the backend to alertmanager API.
With this change fetch() calls in the browser will use those headers if proxy mode is not enabled.
This enables second mode for titlebar styling - enabling it will cause the background to be set to the same color as the alerts, but only if all alerts in a group are in the same state.
Merge into a single function since both className & style are tied together and as such are better of if selected using a single logic. Fixes some minor UI glitches like ellipsis color on truncated labels with custom color