It's odd given that OsmAnd is the most feature complete OSM app for Android that there are so few custom map styles for it. I know the weird format it uses is a contributing factor (I think everyone else uses Mapnik compatible styles). But I find all the included themes really hideous and haven't found a good alternative. There are really good raster styles for outdoor use, so if anyone has advice on adapting an existing theme for OsmAnd I'd be curious to hear about that.