JSON Theme background color not apply

Hi Anonymous, 

In your scenario, the "background" means "The background applies to button fill and combo chart label background. How these colors are used depends on the specific visual style that's applied. The table and matrix visuals apply these styles by default." according to this document. 

In your scenario, if you want to set the page background color via custom theme, you can modify your json file like below: 

{	
    "name": "Custom",
    "dataColors": ["#EE3322", "#FFA300", "#FFEE22", "#C3D600", "#119988", "#7B7EAB", "#B42573", "#998877"],
    "background": "#FFFFFF",
"visualStyles": {
        "page": {
            "*": {
                "background": [
                    {
                        "color": {
                            "solid": {
                                "color": "#000000"
                            }
                        },
                        "transparency": 0
                    }
                ]
            }
        }
    }
}

Reference: 

https://www.cathrinewilhelmsen.net/2018/12/11/custom-power-bi-themes-page-background-colors/

Best Regards,
Qiuyun Yu