Newsfeed styles are not loaded in the new platform

[flash1293]

Kibana version: master

Describe the bug: The newsfeed flyout depends on a few css classes defined here: https://github.com/flash1293/kibana/blob/a50dbefb62edf0be6b98110023afc32b04c06a7f/src/legacy/core_plugins/newsfeed/public/np_ready/components/header_alert/_index.scss

As these styles live in the legacy world and are not imported from within the new platform anywhere, they don't get loaded when only accessing a new platform application (like the logs app).

This problem will get worse once #63443 is merged as it's moving Discover, Visualize and Dashboard to the new platform as well.

[elasticmachine]

Pinging @elastic/kibana-core-ui (Team:Core UI)

[ryankeairns]

@cchaos any thoughts on how we should handle this one?

[flash1293]

From the technical side - these styles should be moved to the new platform and loaded via import in the component using them (indirectly in this case). This is making sure the styles will always be available if the component is around.

[cchaos]

I think this was built before we were able to release EuiHeaderAlert in EUI and so Kibana's implementation is a shim for the EUI component. But now Kibana has access to EuiHeaderAlert, and should use that one instead of this custom implementation. https://elastic.github.io/eui/#/layout/header (bottom section)

[ryankeairns]

@flash1293 @cchaos did @afharo 's PR fix this issue too? #66084

[afharo]

Actually, in that PR we entirely removed that file because it was replaced by an eui component. The CSS files for that component were setting the exact the same values as the defaults from eui.
I'd say this issue can be closed :)

[cqliu1]

Closed with #66084.