Describe the bug
Border top of navigation side-bar is inconsistent between light and dark theme.

Impact and severity
Depends on whether this is a bug or is implemented this way by design.

To Reproduce
Steps to reproduce the behavior:

1. Switch between dark and light themes

Expected behavior
The border, on dark theme, to be identical (lighter) to the right border of the side-bar, as it is on light theme.

Minimum reproducible sandbox
This can be reproduced in the documentation.

Screenshots

Additional context
It seems that the code related to this issue comes from the /eui/packages/eui/src/components/header/header.styles.ts where there is a separate logic for both themes. In the DARK theme the backgroundColor is used to define the bottom border color, whereas for the default theme the defaultBorderColor is used.

There is also a comment about this difference.

/**
 * The `dark` header is (currently) a bit of a special case. We don't
 * actually want to use <EuiThemeProvider colorMode="dark"> inside it
 * because that will affect popovers and `SelectableSitewideTemplate`
 * as well, which we do not necessarily want to do (?)
 *
 * It's also possible that the dark header will go away or become unused
 * by Kibana in the near future, at which point we can remove this
 */

This issue was previously opened on Kibana's side, but I’d like to transfer the issue to EUI repo so that the team could check if this is intentional or if this is a bug.