Please read this instruction:

When you start to change any of the font style settings (1), the temporary template "Custom" (2) becomes active with the settings that were previously installed in it. You can set it to the desired parameters and click on the Heart icon(3) to save this template under a new name (4).
After that, you can simply change the parameters of your new template (5), and all cells with this template will be changed automatically(6).