Every day I work at least 5 different Qradar system and sometimes difficult to follow where I'm now. Is the a possibility to customize the GUI Header IBM Qradar part to differentiate the systems?
you can try this: https://www.robertrojek.pl/2019/03/28/customising-qradar-interface/ but be aware, IBM does not give support for this so I suggest doing it on a non-production system first
Analyst Custom Searches and Toolbox app allows to write your own CSS and inject it into QRadar.
IBM Application Exchange - Analyst Custom Searches and Toolbox for QRadar - v7.5.0 UP7+
Each user can decide for themselves whether they want to use this custom CSS.
Page 36 in the documentation explains how to use it: https://apps.xforce.ibmcloud.com/api/hub/extensionsNew/3df35f6e72eeae484354e266ef9b682e/AnalystCustomSearches_2.0.0.pdf