In case you wish the logo in the affiliate login page to be centered, then you can do it the following way:
In the merchant panel navigate to Configuration > Design > Affiliate panel theme
> click 'Edit theme' at the current theme.
If it claims to create a 'Copy of .... theme', then click yes.
Now you can look up the custom.css file and place there the following code.
Then exit the theme editor and click the 'Set as default' button at the freshly created and edited theme.