Different Font Sizes to Customised Theme

Hi, Anonymous 

I am glad to help you.

It is indeed possible to set the font and size of chart titles and data labels in Power BI.

Unless the visuals are the same type of visual, in which case you can use Ctrl+A to select all the visuals and then set the font to Arial and the size to 12 for both the chart titles and the data labels:

However, in an entire report, there will generally be multiple different types of visuals, so you can use any text editor (such as Notepad or Visual Studio Code) to create or edit the JSON file:

{
  "name": "Custom Theme",
  "visualStyles": {
    "*": {
      "*": {
        "title": [{
          "fontSize": 12,
          "fontFamily": "Arial"
        }],
        "labels": [{
          "fontSize": 12,
          "fontFamily": "Arial"
        }]
      }
    }
  }
}

After saving the JSON file, in Power BI click View, then click Browse for themes, select the JSON file you just saved and apply it, you will find that all the fonts and sizes of the different types of visuals have been changed to the results you need:

I hope my suggestions give you good ideas, if you have any more questions, please clarify in a follow-up reply.
Best Regards,
Fen Ling,
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