Default theme changes

I have created my onw custom theme with some minot font changes but I’m unable to change the default theme in settings. How to do that.

Please use screen capture software to show screen images, clipped to show the relevant part of the screen otherwise they are hard to read.

Themes must be written, activated, and applied. Have you applied your theme to the documents you want to use it on?

see
Themes: Change the look of forms with themes | Set a theme for customer or supplier statements |

New default themes must be set for each transaction type under Form Defaults. See https://www.manager.io/guides/14059.