This is an issue that novice users will have when trying to modify Bartik for use as a custom theme. Bartik, when copied, re-named and placed in mysite.com/themes/custom/new-theme-name, loses access to the Search box "magnifying glass" icon at least, and probably numerous others. A solution to this problem is to also copy the */core/misc folder and place it as */themes/misc so that the new custom theme will be able to access it. Someone may know of a way to more easily connect a path back to the misc folder in core, but I wasn't able to succeed with attempts to do this. A copy placed under the root Themes solved my issue.
Currently the bartik.info.yml file states near the top:
# . . . If you wish to modify the output or assets of Bartik you can:
# 1. Copy the whole of Bartik and rename it as your own theme. You will need to
# manually manage your own updates if you want to stay up to date with Bartik's
# bug fixes and feature support.A novice user reading this documentation won't realize that the core/misc/icons folder is also required to make Bartik work as a new theme. I recommend updating the documentation to reflect this issue. I've not been able to find any other related issue to this in the Drupal 8.x issues queue at this time. Thanks to VM for helping troubleshoot this issue at this related forum topic → .
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Now starter kit theme is available in drupal 8, 9 and 10.
and if wants to create a new subtheme of bartik and add "starterkit: true" in info file and rub following command.
php core/scripts/drupal generate-theme --starterkit bartik my_new_theme
But this is also not resolving icons mission issue so need to copy "/core/misc/icons/" in new theme.
Bartik has been removed from core, moving this to the contrib project.