@ricsca I'm unable to understand your question / issue.
For me all seems work fine.
In my install: I don't see any issue on front end login https://helpdesk.peopleinside.it/it/usr/login
If you mean in the back end operator login the login button can have a different color, maybe but also the left side with the UVdesk logo is not maybe customizable, this is normal I think and is not an issue.
You can always maybe customize CSS in branding section with your own custom code.
Please use screenshot if you need report some issue because not always is easy understand from here.
In the operator (member) login is true that title and login button can have different colour but this is currently not a screen that can be customized.
Maybe can be a bug or something to implement but my suggestion is currently to focus on what is really important for you as there is a lot of fix or improvement that need maybe to be done before this 
Maybe is better to give priority to issues more strong than this that is pending in GitHub.
The work to do is big and there is many things that can be improved, like in all software.
If you find a bug you can report and track on GitHub:
https://github.com/uvdesk/community-skeleton/issues
but as fix require time, also filling a bug should require time and have all details possible.