Default keys are indeed the minus and plus sign. When you want to know a shortcut you can try to check the right-click options. For instance, if you have a frame selected and right-click on it you can go to ‘Exposure’ where your first options on the menu are Increase Exposure and Decrease Exposure and there you’ll see the shortcuts. You can also go to Preferences (Edit > Preferences on Win, Toon Boom Harmony > Preferences in Mac), find any shortcut and change it if you want. Sometimes searching is not immediate if you fail to guess the name of the operation (in this case “exposure”, “increase” or “decrease” would get you there).