How do I now change the size or color of these texts?
Thanks for your help!
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At this time the feature to customize headers and footers doesn't include a way to set a custom formatting (font, color, size, effects like bold, italics, etc.).
One alternative is to use rows to hold the header / footer text and use the cell formatting or in cell text formatting to customize the "headers" and "footers"
Related Q&A: Adding a header and footer to a Google Spreadsheet
Related Help Article: Add headers, footers & page numbers