Font family in button text

Hey. I know this is old, but I wanted to provide the solution as this was the only thread I found while googling.

You have to specify the JSON class name "lightLabel", as this changes button text fonts. Here is a working example:

{
    "name": "CustomTheme",
    "textClasses": {
        "lightLabel": {
            "fontSize": 45,
            "fontFace": "Harlow Solid",
            "color": "#252423"
        }
    }
}

You can read more and find examples here: https://docs.microsoft.com/en-us/power-bi/create-reports/desktop-report-themes#report-theme-json-file-format