The hassle with premade countertops is that they are square!
See if they offer a section of backsplash that you might cut and attach on the side wall that may cover the gap especially if the counter overhangs the cabinet a little bit, it would be thick enough.
As to the side wall below, cut yourself a shim to fill in that space. Replicating the finish may take some trial and error. Some cabinets offer an optional strip you can buy as a filler for that purpose. YOu would need to either plane or cut it to fit.
Things can get a bit more messy if you're dealing with a tub or shower. Unless you're putting in a custom shower that you can make whatever shape needed, you're going to have problems there as well.