[Typo3] Logon image and bg color of stripe at logon

[Carlos Rodriguez]

Hi folks,

I wonder if it is posible to change the logon image (www.abc.def/typo3) 
and the bg color.

Yes I know it is possible just in the code - but thats *not* What I want.
The problem is, when updating to, lets say, 3.8.1 everything is gone.

*How* can I change the loginimage and the bg color of the colored stripe 
, that the changes will *not* be overwritten, when upgrading typo3.

What do I miss in this particulary matter?

Thanks