Hi Francisco,
The format has changed slightly, as the settings were revised to store multiple themes.
See below for the update, note the change of theme from object to array and the isEnabled property:
"webformInfo": {
"themes": [
{
"isEnabled": true,
"webpage": {
"backgroundColor": "#FFFF00"
}
}
]
}
ed. (Apr. 8, 2019) - revised to reflect change of themes from object to array