How to edit CSS of Default search box in multiple pages at a time in sharepoint online

Occasional Visitor

searchbox.PNG
Default search box is appearing as above image in document-set pages of a SharePoint online site.
I can able to fix it on Run time(developer tool) by doing  changes in  .ms-srch-sb but i am not able to find exact CSS file to make these changes and fix it permanently.
Please help.
Thanks
Priti

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