You can enable the Fusion Page Builder for the CartFlows steps as shown in the following Screenshot –
Also, following are the steps where you can find the settings –
Step 1: Go to Settings > Fusion
Step 2: Under the Post Types option enable the Steps
Step 3: Save Changes and the Fusion page builder's Edit with Fusion Builder button will be enabled on the CartFlows steps.
Now, you can design your CartFlows Steps using the Fusion page builder.