Change Power bi focus mode background color

I created my backrounds using PowerPoint and save as SVG files.  These give me a higher image definition.  I create customised icons as PNG files the same way, although they have to be converted to Base64 before they can be using in the JSON script.  If you want to modify a theme using JSON then I suggest starting with a Power BI report where you may have already modified the theme, or just start with one of the default ones.  Go to the View Menu and click on the Theme dropdown list.  Select a preferred theme unless you want to use the current one and click on Save Current Theme.  You can then edit it using a tool like Visual Studio.