Change table design with JSON theme

Hello -

i have the challenge to modify some colors to match our CI guidelines - using tables and matrix in Power BI i need to change the background color of the column headers. I tried the following JSON input:

{
"name":"test design",
"dataColors": [ "#004459", "#54B7C6", "#AB0953", "#BAC405", "#667C72", "#633A11", "#006686", "#AB0953" ],
"background":"#F5F8FC",
"foreground":"#003366",
"tableAccent":"#214F80",

"visualStyles": {

	"tableEx": {
			"*": {	
				"columnHeaders": [{
				            "backColor":"#004459",
				            "fontColor":"#ffffff",
							"wordWrap":false,
							"fontSize":9,
				            "urlIcon":true
                            }],
					"values": [{
							"wordWrap":false,
							"fontSize":8,
						    "urlIcon":true
                            }]
						}
				}
			}
	}

Syntax seems to be ok - font size and other parameters changing - "backColor" is not working at all. Any thoughts what i'm doing wrong or is it a bug?

Many thanks and best regards,

Stephan