office365 - Excel: change default template (book.xltx) - Stack Overflow

That really shouldn't be so difficult...

The issue: I have a customized Excel default template (font: Arial 10.5pt; corporate color palette). When opening Excel (blank workbook) it automatically loads, also when then adding new sheet (clicking on the + to add new sheet).

However, if I try to create a new blank workbook from within Excel (Ctrl-N; or -> File -> New -> Blank workbook), then it reverts back to the standard Excel template (font: Aptos Narrow 11, 11pt; Excel color palette). Funny enough, when then adding a new sheet (Sheet 2) to the file (with etong fonts/color palette on Sheet 1), it uses the correct modified template (from sheet.xltx), with my custom font, but not the color palette (as this is for the whole workbook).

What I want to achieve: Load custom default template also from within Excel (Ctrl-N), without the need to select as a custom template under personal templates.

Questions:

1. Where do I need to place the customised default templates (book.xltx, and sheet.xltx)?
2. Do they need other names?
3. Do I need to modify the Registry to ensure they load even if creating a new file within Excel)?

What I did:

• created a new Excel default template (book.xltx, and sheet.xltx)
• Placed the templates in application Startup path (C:\User[username]\AppData\Roaming\Microsoft\Excel\XLSTART)
• Placed the templates in the "Application template folder" (C:\Program Files\Microsoft Office\root\Templates, then under the correct locale), even though this folder is only for standard templates.

OS: Windows 11 Office: Office365 (Microsoft 365 for enterprise), latest version

For illustration:

1. New workbook loading correct template at Excel startup: Using correct Excel default template
2. New workbook (sheet 1) created through Ctrl-N within Excel: loading wrong default template: Using wrong (standard Excel) default template
3. New sheet within "wrong" blank Excel file: apparently loading correct modified sheet.xltx template: new sheet added