You can do the following to make the background transparent and also Fit To Width.
var embedConfig = {
type: type,
id: id,
embedUrl: embedUrl,
viewMode: getViewModeFromModel(viewMode),
tokenType: models.TokenType.Aad,
accessToken: token,
pageView: 'fitToWidth', // applies to Dashboard only; for Report, see below
pageName: pageName,
// See https://microsoft.github.io/PowerBI-JavaScript/interfaces/_node_modules_powerbi_models_dist_models_d_.isettings.html
settings: {
filterPaneEnabled: true,
navContentPaneEnabled: true,
background: models.BackgroundType.Transparent,
// START Report specific settings
layoutType: models.LayoutType.Custom,
customLayout: {
displayOption: models.DisplayOption.FitToWidth
}
// END Report specific settings
}
}