Can I brand the affiliate dashboard?

Yes, on every plan. How you do it depends on your customer accounts.

On new customer accounts

The dashboard is inside Shopify's customer-account area, so it uses the branding that you set for customer accounts in Shopify. You can't add your own CSS to it. To change what the Affiliate page shows:

1. Open Settings > Dashboard in Simple Affiliate.
2. Click Open affiliate extension editor under Affiliate dashboard URL.
3. Change the page in Shopify's customer accounts editor, and save.

On classic customer accounts

The dashboard is a page on your theme, so it uses the look of your theme. To change it:

1. Open Settings > Dashboard in Simple Affiliate.
2. Under Page content, change the Page title, or add Custom HTML.
3. Under Custom CSS, add your own CSS.
4. Save your changes.