STEP 1: Go to Cumulus Preferences -> Category View Sets -> Category Pane. Check the box to "Restrict selectable master categories to" and "$Categories". This removes the extra $Keywords and $Sources from the Category View Pane.
STEP 2: Right-click "$Categories" from the left side navbar, and select "Information". In the view set dropdown, choose "Customize".
STEP 3: This will open up the Preferences pane, make sure you are in the "Standard" view set. Then click "Add", and choose "Container Type [String List]".
STEP 4: While still in Preferences, go to Catalog Settings -> Category Fields -> Container Type. Right-click to enable "Allow user to edit" so that green checkmark shows in "User Editable" column. Click "Apply" and "OK" to go back to Cumulus.
STEP 5: Right-click "$Categories" again, and change Container Type to "no value". Then click the save icon in upper left.
STEP 6: Move your custom category is placed on the master level of your catalog, then right-click it show information. Set Container Type to "Category", and click the save icon. These will be the categories you see in your web client, now that $Categories has been hidden.
STEP 7: Confirm everything is working by opening the web client. The only containers listed should be your custom categories.