try to use below code..
let params = {
name: 'testStyle' /* STYLEDEFINITIONNAME */,
content: { "content" :
{ "params": "",
"image":"",
"backgroundColor":"#d49d13",
"textSize":"normal",
"lineStyle":"dotted",
"displayString":"Series 15",
"fontEmphasisUnderline":false,
"fontEmphasisItalic":false,
"lineThickness":1,
"lineColor":"#ffac20",
"foregroundColor":"#ffdc84",
"fontEmphasisBold":false,
"secondaryBackgroundColor":"#ffb22f"
}
} /* JSON */
};
// no return
Resources["EntityServices"].UpdateStyleDefinition(params);
Also refer... https://community.ptc.com/t5/ThingWorx-Developers/Style-definition-using-scripts/td-p/606413