Implement italic visual subtitles in Power BI Theme JSON file

I need to implement formatting from a style guide to a Power BI theme. I managed to add the most usual stuff like colours, label formatting and largeTitle formatting.

However, I can't find how to format subtitles of visuals such that they are italic by default. Does anyone know if that is possible at all and if so, how I can fix this? The subTitle part below is not working. The largeTitle is working fine (including the Bold formatting).

{
    "name": "MyCompanyTheme",
    "dataColors": ["#000000", "#00000"],    
    "textClasses": {
        "label": {
            "fontFace": "Segoe UI",            
            "fontSize": 12,
            "color": "#000000"
        },
        "largeTitle": {
            "fontFace": "Segoe UI",
            "fontWeight": "Bold",
            "fontSize": 12,            
            "color": "#001F78"
        },
        "subtitle": {
            "fontFace": "Segoe UI",
            "fontSize": 10,
            "fontStyle": "italic",
            "color": "#000000"
        }     
    },
    "visualStyles": {
    "*": {
        "*": {
            "legend": [
                {
                    "showTitle": false
                }
            ]
        }
    }
}
}

I found an overview of classes here: https://learn.microsoft.com/en-us/power-bi/create-reports/desktop-report-themes. But that list seems not the full list of possible items that can be used.

Many thanks in advance!