Description

The expected behavior for EuiSelectableSitewide used inside EuiHeader in Kibana changes as part of the Visual Refresh input design changes. Previously for the legacy Amsterdam theme, the search would be styled dark when not focused and light when focused. As part of the Visual Refresh this should change to follow the visible color mode.

⚠ EuiHeader currently does not support proper color mode context. Instead it handles theming via theme prop and applies a visual-only dark theme.
Adding a EuiThemeProvider to apply proper color modes to EuiHeader would be the expected usage but this is currently more complicated as implementation usages are built on top of the visual-only theme-dark mode and we'll need to ensure consumers are aligned.
ℹ Kibana classic and Serverless use different variants of the header-search composition.

Toggle screenshots

mode	LIGHT	DARK
classic
classic
serverless

As first iteration we can add a theme prop and EuiThemeProvider to EuiSelectableTemplateSitewide which would allow for a more granular control of the output - similar to EuiHeader - and support the expected output for Visual Refresh by using the actual input styles without adding more custom style overwrites.