We have created a cached map service of Ordnance Survey data, which is a mixture of raster and vector data. When zoomed in past 1:5000 it uses MasterMap vector data. The cache is created in mixed format and for these scales it has a transparent background. When this is added to ArcMap or any desktop product it looks great, but in AGOL or Portal it looks terrible as the canvas colour for these porducts, for some reson, is grey, not white.
Does anyone know if it is possible to change the default colour of the canvas in AGOL? ESRI UK's solution was to put a white polygon behind the data, but this forces it to create a jpeg cache, which is of poor quality and much larger than the mixed (png?) version.
The other alternative is to change the css of the app, but that means we need to host it, which isn't ideal.