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
                    }
                }