You cannot white-label the Business App or Sales & Success Center Login pages. However, if you are using multiple markets with different branding on each, you can use a generic logo for the default branding market. This will display the generic logo on the login page, and as soon as the user logs into Business App or Sales & Success Center, the market-specific branding will be shown within Business App or Sales & Success Center.
To change the branding, please go to Administration > Partner Branding > Logo.
If you have multiple markets and would like to brand them differently, select another Market in the 'All Markets' tab (top right of the same page) and customize accordingly.